diff --git a/src/038-unions-and-narrowing/037-introduction-to-unions.problem.ts b/src/018-unions-and-narrowing/037-introduction-to-unions.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/037-introduction-to-unions.problem.ts
rename to src/018-unions-and-narrowing/037-introduction-to-unions.problem.ts
diff --git a/src/038-unions-and-narrowing/037-introduction-to-unions.solution.ts b/src/018-unions-and-narrowing/037-introduction-to-unions.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/037-introduction-to-unions.solution.ts
rename to src/018-unions-and-narrowing/037-introduction-to-unions.solution.ts
diff --git a/src/038-unions-and-narrowing/038-literal-types.problem.ts b/src/018-unions-and-narrowing/038-literal-types.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/038-literal-types.problem.ts
rename to src/018-unions-and-narrowing/038-literal-types.problem.ts
diff --git a/src/038-unions-and-narrowing/038-literal-types.solution.1.ts b/src/018-unions-and-narrowing/038-literal-types.solution.1.ts
similarity index 100%
rename from src/038-unions-and-narrowing/038-literal-types.solution.1.ts
rename to src/018-unions-and-narrowing/038-literal-types.solution.1.ts
diff --git a/src/038-unions-and-narrowing/038-literal-types.solution.2.ts b/src/018-unions-and-narrowing/038-literal-types.solution.2.ts
similarity index 100%
rename from src/038-unions-and-narrowing/038-literal-types.solution.2.ts
rename to src/018-unions-and-narrowing/038-literal-types.solution.2.ts
diff --git a/src/038-unions-and-narrowing/039-combining-unions.problem.ts b/src/018-unions-and-narrowing/039-combining-unions.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/039-combining-unions.problem.ts
rename to src/018-unions-and-narrowing/039-combining-unions.problem.ts
diff --git a/src/038-unions-and-narrowing/039-combining-unions.solution.ts b/src/018-unions-and-narrowing/039-combining-unions.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/039-combining-unions.solution.ts
rename to src/018-unions-and-narrowing/039-combining-unions.solution.ts
diff --git a/src/038-unions-and-narrowing/040-how-big-can-a-union-be.explainer.ts b/src/018-unions-and-narrowing/040-how-big-can-a-union-be.explainer.ts
similarity index 100%
rename from src/038-unions-and-narrowing/040-how-big-can-a-union-be.explainer.ts
rename to src/018-unions-and-narrowing/040-how-big-can-a-union-be.explainer.ts
diff --git a/src/038-unions-and-narrowing/041-literals-vs-wider-types.explainer.ts b/src/018-unions-and-narrowing/041-literals-vs-wider-types.explainer.ts
similarity index 100%
rename from src/038-unions-and-narrowing/041-literals-vs-wider-types.explainer.ts
rename to src/018-unions-and-narrowing/041-literals-vs-wider-types.explainer.ts
diff --git a/src/038-unions-and-narrowing/042-narrowing-unions-with-typeof.explainer.ts b/src/018-unions-and-narrowing/042-narrowing-unions-with-typeof.explainer.ts
similarity index 100%
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rename to src/018-unions-and-narrowing/042-narrowing-unions-with-typeof.explainer.ts
diff --git a/src/038-unions-and-narrowing/043-narrowing-with-if-statements.problem.ts b/src/018-unions-and-narrowing/043-narrowing-with-if-statements.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/043-narrowing-with-if-statements.problem.ts
rename to src/018-unions-and-narrowing/043-narrowing-with-if-statements.problem.ts
diff --git a/src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.1.ts b/src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.1.ts
similarity index 100%
rename from src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.1.ts
rename to src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.1.ts
diff --git a/src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.2.ts b/src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.2.ts
similarity index 100%
rename from src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.2.ts
rename to src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.2.ts
diff --git a/src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.3.ts b/src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.3.ts
similarity index 100%
rename from src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.3.ts
rename to src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.3.ts
diff --git a/src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.4.ts b/src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.4.ts
similarity index 100%
rename from src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.4.ts
rename to src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.4.ts
diff --git a/src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.5.ts b/src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.5.ts
similarity index 100%
rename from src/038-unions-and-narrowing/043-narrowing-with-if-statements.solution.5.ts
rename to src/018-unions-and-narrowing/043-narrowing-with-if-statements.solution.5.ts
diff --git a/src/038-unions-and-narrowing/044-narrowing-with-boolean-wont-work.explainer.ts b/src/018-unions-and-narrowing/044-narrowing-with-boolean-wont-work.explainer.ts
similarity index 100%
rename from src/038-unions-and-narrowing/044-narrowing-with-boolean-wont-work.explainer.ts
rename to src/018-unions-and-narrowing/044-narrowing-with-boolean-wont-work.explainer.ts
diff --git a/src/038-unions-and-narrowing/045-throwing-errors-to-narrow.problem.ts b/src/018-unions-and-narrowing/045-throwing-errors-to-narrow.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/045-throwing-errors-to-narrow.problem.ts
rename to src/018-unions-and-narrowing/045-throwing-errors-to-narrow.problem.ts
diff --git a/src/038-unions-and-narrowing/045-throwing-errors-to-narrow.solution.ts b/src/018-unions-and-narrowing/045-throwing-errors-to-narrow.solution.ts
similarity index 100%
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rename to src/018-unions-and-narrowing/045-throwing-errors-to-narrow.solution.ts
diff --git a/src/038-unions-and-narrowing/046-narrowing-with-instanceof-statements.problem.ts b/src/018-unions-and-narrowing/046-narrowing-with-instanceof-statements.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/046-narrowing-with-instanceof-statements.problem.ts
rename to src/018-unions-and-narrowing/046-narrowing-with-instanceof-statements.problem.ts
diff --git a/src/038-unions-and-narrowing/046-narrowing-with-instanceof-statements.solution.ts b/src/018-unions-and-narrowing/046-narrowing-with-instanceof-statements.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/046-narrowing-with-instanceof-statements.solution.ts
rename to src/018-unions-and-narrowing/046-narrowing-with-instanceof-statements.solution.ts
diff --git a/src/038-unions-and-narrowing/047-narrowing-with-in-statements.problem.ts b/src/018-unions-and-narrowing/047-narrowing-with-in-statements.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/047-narrowing-with-in-statements.problem.ts
rename to src/018-unions-and-narrowing/047-narrowing-with-in-statements.problem.ts
diff --git a/src/038-unions-and-narrowing/047-narrowing-with-in-statements.solution.ts b/src/018-unions-and-narrowing/047-narrowing-with-in-statements.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/047-narrowing-with-in-statements.solution.ts
rename to src/018-unions-and-narrowing/047-narrowing-with-in-statements.solution.ts
diff --git a/src/038-unions-and-narrowing/048-narrowing-unknown-to-a-value.problem.ts b/src/018-unions-and-narrowing/048-narrowing-unknown-to-a-value.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/048-narrowing-unknown-to-a-value.problem.ts
rename to src/018-unions-and-narrowing/048-narrowing-unknown-to-a-value.problem.ts
diff --git a/src/038-unions-and-narrowing/048-narrowing-unknown-to-a-value.solution.ts b/src/018-unions-and-narrowing/048-narrowing-unknown-to-a-value.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/048-narrowing-unknown-to-a-value.solution.ts
rename to src/018-unions-and-narrowing/048-narrowing-unknown-to-a-value.solution.ts
diff --git a/src/038-unions-and-narrowing/049-narrowing-with-ternaries.problem.ts b/src/018-unions-and-narrowing/049-narrowing-with-ternaries.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/049-narrowing-with-ternaries.problem.ts
rename to src/018-unions-and-narrowing/049-narrowing-with-ternaries.problem.ts
diff --git a/src/038-unions-and-narrowing/049-narrowing-with-ternaries.solution.ts b/src/018-unions-and-narrowing/049-narrowing-with-ternaries.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/049-narrowing-with-ternaries.solution.ts
rename to src/018-unions-and-narrowing/049-narrowing-with-ternaries.solution.ts
diff --git a/src/038-unions-and-narrowing/050-narrowing-with-nullish-coalescing.problem.ts b/src/018-unions-and-narrowing/050-narrowing-with-nullish-coalescing.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/050-narrowing-with-nullish-coalescing.problem.ts
rename to src/018-unions-and-narrowing/050-narrowing-with-nullish-coalescing.problem.ts
diff --git a/src/038-unions-and-narrowing/050-narrowing-with-nullish-coalescing.solution.ts b/src/018-unions-and-narrowing/050-narrowing-with-nullish-coalescing.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/050-narrowing-with-nullish-coalescing.solution.ts
rename to src/018-unions-and-narrowing/050-narrowing-with-nullish-coalescing.solution.ts
diff --git a/src/038-unions-and-narrowing/051-intro-to-discriminated-unions.problem.ts b/src/018-unions-and-narrowing/051-intro-to-discriminated-unions.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/051-intro-to-discriminated-unions.problem.ts
rename to src/018-unions-and-narrowing/051-intro-to-discriminated-unions.problem.ts
diff --git a/src/038-unions-and-narrowing/051-intro-to-discriminated-unions.solution.ts b/src/018-unions-and-narrowing/051-intro-to-discriminated-unions.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/051-intro-to-discriminated-unions.solution.ts
rename to src/018-unions-and-narrowing/051-intro-to-discriminated-unions.solution.ts
diff --git a/src/038-unions-and-narrowing/052-destructuring-a-discriminated-union.problem.ts b/src/018-unions-and-narrowing/052-destructuring-a-discriminated-union.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/052-destructuring-a-discriminated-union.problem.ts
rename to src/018-unions-and-narrowing/052-destructuring-a-discriminated-union.problem.ts
diff --git a/src/038-unions-and-narrowing/052-destructuring-a-discriminated-union.solution.ts b/src/018-unions-and-narrowing/052-destructuring-a-discriminated-union.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/052-destructuring-a-discriminated-union.solution.ts
rename to src/018-unions-and-narrowing/052-destructuring-a-discriminated-union.solution.ts
diff --git a/src/038-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.problem.ts b/src/018-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.problem.ts
rename to src/018-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.problem.ts
diff --git a/src/038-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.solution.ts b/src/018-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.solution.ts
rename to src/018-unions-and-narrowing/053-narrowing-a-discriminated-union-with-a-switch-statement.solution.ts
diff --git a/src/038-unions-and-narrowing/054-narrowing-with-switch-true.explainer.ts b/src/018-unions-and-narrowing/054-narrowing-with-switch-true.explainer.ts
similarity index 100%
rename from src/038-unions-and-narrowing/054-narrowing-with-switch-true.explainer.ts
rename to src/018-unions-and-narrowing/054-narrowing-with-switch-true.explainer.ts
diff --git a/src/038-unions-and-narrowing/055-destructuring-a-discriminated-tuple.problem.ts b/src/018-unions-and-narrowing/055-destructuring-a-discriminated-tuple.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/055-destructuring-a-discriminated-tuple.problem.ts
rename to src/018-unions-and-narrowing/055-destructuring-a-discriminated-tuple.problem.ts
diff --git a/src/038-unions-and-narrowing/055-destructuring-a-discriminated-tuple.solution.ts b/src/018-unions-and-narrowing/055-destructuring-a-discriminated-tuple.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/055-destructuring-a-discriminated-tuple.solution.ts
rename to src/018-unions-and-narrowing/055-destructuring-a-discriminated-tuple.solution.ts
diff --git a/src/038-unions-and-narrowing/056-discriminated-booleans.problem.ts b/src/018-unions-and-narrowing/056-discriminated-booleans.problem.ts
similarity index 100%
rename from src/038-unions-and-narrowing/056-discriminated-booleans.problem.ts
rename to src/018-unions-and-narrowing/056-discriminated-booleans.problem.ts
diff --git a/src/038-unions-and-narrowing/056-discriminated-booleans.solution.ts b/src/018-unions-and-narrowing/056-discriminated-booleans.solution.ts
similarity index 100%
rename from src/038-unions-and-narrowing/056-discriminated-booleans.solution.ts
rename to src/018-unions-and-narrowing/056-discriminated-booleans.solution.ts
diff --git a/src/020-sets-of-types/057-extend-object-using-intersections.problem.ts b/src/020-sets-of-types/057-extend-object-using-intersections.problem.ts
new file mode 100644
index 0000000..f7dacd1
--- /dev/null
+++ b/src/020-sets-of-types/057-extend-object-using-intersections.problem.ts
@@ -0,0 +1,38 @@
+type User = {
+  id: string;
+  createdAt: Date;
+  name: string;
+  email: string;
+};
+
+type Product = {
+  id: string;
+  createdAt: Date;
+  name: string;
+  price: number;
+};
+
+type tests = [
+  Expect<
+    Extends<
+      User,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        email: string;
+      }
+    >
+  >,
+  Expect<
+    Extends<
+      Product,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        price: number;
+      }
+    >
+  >,
+];
diff --git a/src/020-sets-of-types/057-extend-object-using-intersections.solution.ts b/src/020-sets-of-types/057-extend-object-using-intersections.solution.ts
new file mode 100644
index 0000000..f329d64
--- /dev/null
+++ b/src/020-sets-of-types/057-extend-object-using-intersections.solution.ts
@@ -0,0 +1,39 @@
+type BaseEntity = {
+  id: string;
+  createdAt: Date;
+};
+
+type User = {
+  name: string;
+  email: string;
+} & BaseEntity;
+
+type Product = {
+  name: string;
+  price: number;
+} & BaseEntity;
+
+type tests = [
+  Expect<
+    Extends<
+      User,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        email: string;
+      }
+    >
+  >,
+  Expect<
+    Extends<
+      Product,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        price: number;
+      }
+    >
+  >,
+];
diff --git a/src/020-sets-of-types/058-extend-object-using-interfaces.problem.ts b/src/020-sets-of-types/058-extend-object-using-interfaces.problem.ts
new file mode 100644
index 0000000..f329d64
--- /dev/null
+++ b/src/020-sets-of-types/058-extend-object-using-interfaces.problem.ts
@@ -0,0 +1,39 @@
+type BaseEntity = {
+  id: string;
+  createdAt: Date;
+};
+
+type User = {
+  name: string;
+  email: string;
+} & BaseEntity;
+
+type Product = {
+  name: string;
+  price: number;
+} & BaseEntity;
+
+type tests = [
+  Expect<
+    Extends<
+      User,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        email: string;
+      }
+    >
+  >,
+  Expect<
+    Extends<
+      Product,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        price: number;
+      }
+    >
+  >,
+];
diff --git a/src/020-sets-of-types/058-extend-object-using-interfaces.solution.1.ts b/src/020-sets-of-types/058-extend-object-using-interfaces.solution.1.ts
new file mode 100644
index 0000000..b5c7668
--- /dev/null
+++ b/src/020-sets-of-types/058-extend-object-using-interfaces.solution.1.ts
@@ -0,0 +1,39 @@
+interface BaseEntity {
+  id: string;
+  createdAt: Date;
+}
+
+interface User extends BaseEntity {
+  name: string;
+  email: string;
+}
+
+interface Product extends BaseEntity {
+  name: string;
+  price: number;
+}
+
+type tests = [
+  Expect<
+    Extends<
+      User,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        email: string;
+      }
+    >
+  >,
+  Expect<
+    Extends<
+      Product,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        price: number;
+      }
+    >
+  >,
+];
diff --git a/src/020-sets-of-types/058-extend-object-using-interfaces.solution.2.ts b/src/020-sets-of-types/058-extend-object-using-interfaces.solution.2.ts
new file mode 100644
index 0000000..d02ee95
--- /dev/null
+++ b/src/020-sets-of-types/058-extend-object-using-interfaces.solution.2.ts
@@ -0,0 +1,42 @@
+interface WithId {
+  id: string;
+}
+
+interface WithCreatedAt {
+  createdAt: Date;
+}
+
+interface User extends WithId, WithCreatedAt {
+  name: string;
+  email: string;
+}
+
+interface Product extends WithId, WithCreatedAt {
+  name: string;
+  price: number;
+}
+
+type tests = [
+  Expect<
+    Extends<
+      User,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        email: string;
+      }
+    >
+  >,
+  Expect<
+    Extends<
+      Product,
+      {
+        id: string;
+        createdAt: Date;
+        name: string;
+        price: number;
+      }
+    >
+  >,
+];
diff --git a/src/020-sets-of-types/059-compare-between-intersections-and-interfaces.explainer.ts b/src/020-sets-of-types/059-compare-between-intersections-and-interfaces.explainer.ts
new file mode 100644
index 0000000..1e1fc55
--- /dev/null
+++ b/src/020-sets-of-types/059-compare-between-intersections-and-interfaces.explainer.ts
@@ -0,0 +1 @@
+// https://github.com/microsoft/TypeScript/wiki/Performance#preferring-interfaces-over-intersections
diff --git a/src/020-sets-of-types/060-index-signatures.problem.ts b/src/020-sets-of-types/060-index-signatures.problem.ts
new file mode 100644
index 0000000..6e2fd48
--- /dev/null
+++ b/src/020-sets-of-types/060-index-signatures.problem.ts
@@ -0,0 +1,5 @@
+const scores = {};
+
+scores.math = 95;
+scores.english = 90;
+scores.science = 85;
diff --git a/src/020-sets-of-types/060-index-signatures.solution.1.ts b/src/020-sets-of-types/060-index-signatures.solution.1.ts
new file mode 100644
index 0000000..1fa76b7
--- /dev/null
+++ b/src/020-sets-of-types/060-index-signatures.solution.1.ts
@@ -0,0 +1,9 @@
+type Scores = {
+  [key: string]: number;
+};
+
+const scores: Scores = {};
+
+scores.math = 95;
+scores.english = 90;
+scores.science = 85;
diff --git a/src/020-sets-of-types/060-index-signatures.solution.2.ts b/src/020-sets-of-types/060-index-signatures.solution.2.ts
new file mode 100644
index 0000000..c2a9440
--- /dev/null
+++ b/src/020-sets-of-types/060-index-signatures.solution.2.ts
@@ -0,0 +1,9 @@
+interface Scores {
+  [key: string]: number;
+}
+
+const scores: Scores = {};
+
+scores.math = 95;
+scores.english = 90;
+scores.science = 85;
diff --git a/src/020-sets-of-types/060-index-signatures.solution.3.ts b/src/020-sets-of-types/060-index-signatures.solution.3.ts
new file mode 100644
index 0000000..a253f5a
--- /dev/null
+++ b/src/020-sets-of-types/060-index-signatures.solution.3.ts
@@ -0,0 +1,7 @@
+const scores: {
+  [key: string]: number;
+} = {};
+
+scores.math = 95;
+scores.english = 90;
+scores.science = 85;
diff --git a/src/020-sets-of-types/060-index-signatures.solution.4.ts b/src/020-sets-of-types/060-index-signatures.solution.4.ts
new file mode 100644
index 0000000..0d6c36e
--- /dev/null
+++ b/src/020-sets-of-types/060-index-signatures.solution.4.ts
@@ -0,0 +1,5 @@
+const scores: Record<string, number> = {};
+
+scores.math = 95;
+scores.english = 90;
+scores.science = 85;
diff --git a/src/020-sets-of-types/061-index-signatures-with-defined-keys.problem.ts b/src/020-sets-of-types/061-index-signatures-with-defined-keys.problem.ts
new file mode 100644
index 0000000..911cb43
--- /dev/null
+++ b/src/020-sets-of-types/061-index-signatures-with-defined-keys.problem.ts
@@ -0,0 +1,16 @@
+interface Scores {
+  [key: string]: number;
+  math: number;
+  english: number;
+  science: number;
+}
+
+// @ts-expect-error science is missing!
+const scores: Scores = {
+  math: 95,
+  english: 90,
+};
+
+scores.athletics = 100;
+scores.french = 75;
+scores.spanish = 70;
diff --git a/src/020-sets-of-types/061-index-signatures-with-defined-keys.solution.1.ts b/src/020-sets-of-types/061-index-signatures-with-defined-keys.solution.1.ts
new file mode 100644
index 0000000..911cb43
--- /dev/null
+++ b/src/020-sets-of-types/061-index-signatures-with-defined-keys.solution.1.ts
@@ -0,0 +1,16 @@
+interface Scores {
+  [key: string]: number;
+  math: number;
+  english: number;
+  science: number;
+}
+
+// @ts-expect-error science is missing!
+const scores: Scores = {
+  math: 95,
+  english: 90,
+};
+
+scores.athletics = 100;
+scores.french = 75;
+scores.spanish = 70;
diff --git a/src/020-sets-of-types/061-index-signatures-with-defined-keys.solution.2.ts b/src/020-sets-of-types/061-index-signatures-with-defined-keys.solution.2.ts
new file mode 100644
index 0000000..05e6d6e
--- /dev/null
+++ b/src/020-sets-of-types/061-index-signatures-with-defined-keys.solution.2.ts
@@ -0,0 +1,19 @@
+interface RequiredScores {
+  math: number;
+  english: number;
+  science: number;
+}
+
+interface Scores extends RequiredScores {
+  [key: string]: number;
+}
+
+// @ts-expect-error science is missing!
+const scores: Scores = {
+  math: 95,
+  english: 90,
+};
+
+scores.athletics = 100;
+scores.french = 75;
+scores.spanish = 70;
diff --git a/src/020-sets-of-types/062-record-type-with-union-as-keys.problem.ts b/src/020-sets-of-types/062-record-type-with-union-as-keys.problem.ts
new file mode 100644
index 0000000..e69de29
diff --git a/src/020-sets-of-types/062-record-type-with-union-as-keys.solution.ts b/src/020-sets-of-types/062-record-type-with-union-as-keys.solution.ts
new file mode 100644
index 0000000..e69de29
diff --git a/src/020-sets-of-types/063-declaration-merging-of-interfaces.problem.ts b/src/020-sets-of-types/063-declaration-merging-of-interfaces.problem.ts
new file mode 100644
index 0000000..07f1ee8
--- /dev/null
+++ b/src/020-sets-of-types/063-declaration-merging-of-interfaces.problem.ts
@@ -0,0 +1,19 @@
+interface Logger {
+  log(message: string, level: number): void;
+}
+
+interface Logger {
+  log(message: string): void;
+}
+
+const myLogger: Logger = {
+  log: (message: string) => {
+    console.log(message);
+  },
+};
+
+myLogger.log(
+  "My message",
+  // @ts-expect-error Level is NOT needed
+  123,
+);
diff --git a/src/020-sets-of-types/063-declaration-merging-of-interfaces.solution.ts b/src/020-sets-of-types/063-declaration-merging-of-interfaces.solution.ts
new file mode 100644
index 0000000..32cfb33
--- /dev/null
+++ b/src/020-sets-of-types/063-declaration-merging-of-interfaces.solution.ts
@@ -0,0 +1,15 @@
+type Logger = {
+  log(message: string): void;
+};
+
+const myLogger: Logger = {
+  log: (message: string) => {
+    console.log(message);
+  },
+};
+
+myLogger.log(
+  "My message",
+  // @ts-expect-error Level is NOT needed
+  123,
+);
diff --git a/src/020-sets-of-types/064-pick-type-helper.problem.ts b/src/020-sets-of-types/064-pick-type-helper.problem.ts
new file mode 100644
index 0000000..7a0f1c5
--- /dev/null
+++ b/src/020-sets-of-types/064-pick-type-helper.problem.ts
@@ -0,0 +1,18 @@
+interface User {
+  id: string;
+  name: string;
+  email: string;
+  role: string;
+}
+
+const fetchUser = async () => {
+  const response = await fetch("/api/user");
+  const user = await response.json();
+  return user;
+};
+
+const example = async () => {
+  const user = await fetchUser();
+
+  type test = Expect<Equal<typeof user, { name: string; email: string }>>;
+};
diff --git a/src/020-sets-of-types/064-pick-type-helper.solution.1.ts b/src/020-sets-of-types/064-pick-type-helper.solution.1.ts
new file mode 100644
index 0000000..f55a9fa
--- /dev/null
+++ b/src/020-sets-of-types/064-pick-type-helper.solution.1.ts
@@ -0,0 +1,21 @@
+interface NameAndEmail {
+  name: string;
+  email: string;
+}
+
+interface User extends NameAndEmail {
+  id: string;
+  role: string;
+}
+
+const fetchUser = async (): Promise<NameAndEmail> => {
+  const response = await fetch("/api/user");
+  const user = await response.json();
+  return user;
+};
+
+const example = async () => {
+  const user = await fetchUser();
+
+  type test = Expect<Equal<typeof user, { name: string; email: string }>>;
+};
diff --git a/src/020-sets-of-types/064-pick-type-helper.solution.2.ts b/src/020-sets-of-types/064-pick-type-helper.solution.2.ts
new file mode 100644
index 0000000..38c80ad
--- /dev/null
+++ b/src/020-sets-of-types/064-pick-type-helper.solution.2.ts
@@ -0,0 +1,20 @@
+interface User {
+  id: string;
+  name: string;
+  email: string;
+  role: string;
+}
+
+type OnlyNameAndEmail = Pick<User, "name" | "email">;
+
+const fetchUser = async (): Promise<OnlyNameAndEmail> => {
+  const response = await fetch("/api/user");
+  const user = await response.json();
+  return user;
+};
+
+const example = async () => {
+  const user = await fetchUser();
+
+  type test = Expect<Equal<typeof user, { name: string; email: string }>>;
+};
diff --git a/src/020-sets-of-types/065-omit-type-helper.problem.ts b/src/020-sets-of-types/065-omit-type-helper.problem.ts
new file mode 100644
index 0000000..8c3577d
--- /dev/null
+++ b/src/020-sets-of-types/065-omit-type-helper.problem.ts
@@ -0,0 +1,24 @@
+interface Product {
+  id: number;
+  name: string;
+  price: number;
+  description: string;
+}
+
+const addProduct = (productInfo: Product) => {
+  // Do something with the productInfo
+};
+
+addProduct({
+  name: "Book",
+  price: 12.99,
+  description: "A book about Dragons",
+});
+
+addProduct({
+  // @ts-expect-error
+  id: 1,
+  name: "Book",
+  price: 12.99,
+  description: "A book about Dragons",
+});
diff --git a/src/020-sets-of-types/065-omit-type-helper.solution.1.ts b/src/020-sets-of-types/065-omit-type-helper.solution.1.ts
new file mode 100644
index 0000000..f8e6ad1
--- /dev/null
+++ b/src/020-sets-of-types/065-omit-type-helper.solution.1.ts
@@ -0,0 +1,29 @@
+interface WithId {
+  id: number;
+}
+
+interface ProductInfo {
+  name: string;
+  price: number;
+  description: string;
+}
+
+interface Product extends WithId, ProductInfo {}
+
+const addProduct = (productInfo: ProductInfo) => {
+  // Do something with the productInfo
+};
+
+addProduct({
+  name: "Book",
+  price: 12.99,
+  description: "A book about Dragons",
+});
+
+addProduct({
+  // @ts-expect-error
+  id: 1,
+  name: "Book",
+  price: 12.99,
+  description: "A book about Dragons",
+});
diff --git a/src/020-sets-of-types/065-omit-type-helper.solution.2.ts b/src/020-sets-of-types/065-omit-type-helper.solution.2.ts
new file mode 100644
index 0000000..4351a6f
--- /dev/null
+++ b/src/020-sets-of-types/065-omit-type-helper.solution.2.ts
@@ -0,0 +1,24 @@
+interface Product {
+  id: number;
+  name: string;
+  price: number;
+  description: string;
+}
+
+const addProduct = (productInfo: Omit<Product, "id">) => {
+  // Do something with the productInfo
+};
+
+addProduct({
+  name: "Book",
+  price: 12.99,
+  description: "A book about Dragons",
+});
+
+addProduct({
+  // @ts-expect-error
+  id: 1,
+  name: "Book",
+  price: 12.99,
+  description: "A book about Dragons",
+});
diff --git a/src/020-sets-of-types/066-partial-type-helper.problem.ts b/src/020-sets-of-types/066-partial-type-helper.problem.ts
new file mode 100644
index 0000000..7370405
--- /dev/null
+++ b/src/020-sets-of-types/066-partial-type-helper.problem.ts
@@ -0,0 +1,34 @@
+interface Product {
+  id: number;
+  name: string;
+  price: number;
+  description: string;
+}
+
+const updateProduct = (id: number, productInfo: Omit<Product, "id">) => {
+  // Do something with the productInfo
+};
+
+// Should be able to update individual pieces of information
+updateProduct(1, {
+  name: "Book",
+});
+
+updateProduct(1, {
+  price: 12.99,
+});
+
+updateProduct(1, {
+  description: "A book about Dragons",
+});
+
+// Should be able to update more than one piece of info at once
+updateProduct(1, {
+  name: "Book",
+  price: 12.99,
+});
+
+updateProduct(1, {
+  name: "Book",
+  description: "A book about Dragons",
+});
diff --git a/src/020-sets-of-types/066-partial-type-helper.solution.ts b/src/020-sets-of-types/066-partial-type-helper.solution.ts
new file mode 100644
index 0000000..ea9fbf7
--- /dev/null
+++ b/src/020-sets-of-types/066-partial-type-helper.solution.ts
@@ -0,0 +1,37 @@
+interface Product {
+  id: number;
+  name: string;
+  price: number;
+  description: string;
+}
+
+const updateProduct = (
+  id: number,
+  productInfo: Partial<Omit<Product, "id">>,
+) => {
+  // Do something with the productInfo
+};
+
+// Should be able to update individual pieces of information
+updateProduct(1, {
+  name: "Book",
+});
+
+updateProduct(1, {
+  price: 12.99,
+});
+
+updateProduct(1, {
+  description: "A book about Dragons",
+});
+
+// Should be able to update more than one piece of info at once
+updateProduct(1, {
+  name: "Book",
+  price: 12.99,
+});
+
+updateProduct(1, {
+  name: "Book",
+  description: "A book about Dragons",
+});
diff --git a/src/020-sets-of-types/067-required-type-helper.explainer.ts b/src/020-sets-of-types/067-required-type-helper.explainer.ts
new file mode 100644
index 0000000..d8f0c31
--- /dev/null
+++ b/src/020-sets-of-types/067-required-type-helper.explainer.ts
@@ -0,0 +1,8 @@
+type Coordinates = {
+  x?: number;
+  y?: number;
+};
+
+type CoordinatesRequired = Required<Coordinates>;
+
+type test = Expect<Equal<CoordinatesRequired, { x: number; y: number }>>;