diff --git a/Sprint-1/fix/median.js b/Sprint-1/fix/median.js
index 5c5b796e1..9e8e7ab43 100644
--- a/Sprint-1/fix/median.js
+++ b/Sprint-1/fix/median.js
@@ -6,9 +6,18 @@
 // or contains values that aren't numbers (the function is expected to throw - see the tests).
 
 function calculateMedian(list) {
-  const middleIndex = Math.floor(list.length / 2);
-  const median = list.splice(middleIndex, 1)[0];
-  return median;
+  if (!Array.isArray(list) || list.some((item) => typeof item !== "number")) {
+    throw new Error("calculateMedian requires an array of numbers");
+  }
+  if (list.length === 0) {
+    throw new Error("calculateMedian requires a non-empty array");
+  }
+  const sorted = [...list].sort((a, b) => a - b);
+  const middleIndex = Math.floor(sorted.length / 2);
+  if (sorted.length % 2 === 0) {
+    return (sorted[middleIndex - 1] + sorted[middleIndex]) / 2;
+  }
+  return sorted[middleIndex];
 }
 
 module.exports = calculateMedian;
diff --git a/Sprint-1/implement/dedupe.js b/Sprint-1/implement/dedupe.js
index 781e8718a..7abb5fb70 100644
--- a/Sprint-1/implement/dedupe.js
+++ b/Sprint-1/implement/dedupe.js
@@ -1 +1,5 @@
-function dedupe() {}
+function dedupe(list) {
+  return [...new Set(list)];
+}
+
+module.exports = dedupe;
diff --git a/Sprint-1/implement/dedupe.test.js b/Sprint-1/implement/dedupe.test.js
index d7c8e3d8e..a98b76d42 100644
--- a/Sprint-1/implement/dedupe.test.js
+++ b/Sprint-1/implement/dedupe.test.js
@@ -16,7 +16,22 @@ E.g. dedupe([1, 2, 1]) returns [1, 2]
 // Given an empty array
 // When passed to the dedupe function
 // Then it should return an empty array
-test.todo("given an empty array, it returns an empty array");
+test("given an empty array, it returns an empty array", () => {
+  expect(dedupe([])).toEqual([]);
+});
+
+test("given an array with no duplicates, returns a copy", () => {
+  const input = [1, 2, 3];
+  const result = dedupe(input);
+  expect(result).toEqual([1, 2, 3]);
+  expect(result).not.toBe(input);
+});
+
+test("removes duplicates preserving first occurrences", () => {
+  expect(dedupe(["a", "a", "a", "b", "b", "c"])).toEqual(["a", "b", "c"]);
+  expect(dedupe([5, 1, 1, 2, 3, 2, 5, 8])).toEqual([5, 1, 2, 3, 8]);
+  expect(dedupe([1, 2, 1])).toEqual([1, 2]);
+});
 
 // Given an array with no duplicates
 // When passed to the dedupe function
diff --git a/Sprint-1/implement/describe-median.js b/Sprint-1/implement/describe-median.js
index 3381cfc0a..c9ce2385a 100644
--- a/Sprint-1/implement/describe-median.js
+++ b/Sprint-1/implement/describe-median.js
@@ -20,6 +20,12 @@ function calculateMedian(list) {
 }
 
 // Implement this function. See describe-median.test.js for the acceptance criteria.
-function describeMedian(list) {}
+function describeMedian(list) {
+  try {
+    return `The median is ${calculateMedian(list)}`;
+  } catch (error) {
+    return `Could not calculate a median: ${error.message}`;
+  }
+}
 
 module.exports = describeMedian;
diff --git a/Sprint-1/implement/describe-median.test.js b/Sprint-1/implement/describe-median.test.js
index e7d18a264..698cc6cfd 100644
--- a/Sprint-1/implement/describe-median.test.js
+++ b/Sprint-1/implement/describe-median.test.js
@@ -21,7 +21,21 @@ const describeMedian = require("./describe-median.js");
 // When passed to describeMedian
 // Then it should return "The median is " followed by the median
 // Delete this test.todo and replace it with a test.
-test.todo('given [1, 2, 3], returns "The median is 2"');
+test('given [1, 2, 3], returns "The median is 2"', () => {
+  expect(describeMedian([1, 2, 3])).toBe("The median is 2");
+});
+
+test("given an empty array, explains why", () => {
+  expect(describeMedian([])).toBe(
+    "Could not calculate a median: calculateMedian requires a non-empty array"
+  );
+});
+
+test("given a non-array, explains why", () => {
+  expect(describeMedian("banana")).toBe(
+    "Could not calculate a median: calculateMedian requires an array of numbers"
+  );
+});
 
 // Given an empty array
 // When passed to describeMedian
diff --git a/Sprint-1/implement/max.js b/Sprint-1/implement/max.js
index 6dd76378e..74ed6e2bf 100644
--- a/Sprint-1/implement/max.js
+++ b/Sprint-1/implement/max.js
@@ -1,4 +1,15 @@
 function findMax(elements) {
+  if (
+    !Array.isArray(elements) ||
+    elements.some((item) => typeof item !== "number")
+  ) {
+    throw new Error("findMax requires an array of numbers");
+  }
+  let max = -Infinity;
+  for (const item of elements) {
+    if (item > max) max = item;
+  }
+  return max;
 }
 
 module.exports = findMax;
diff --git a/Sprint-1/implement/max.test.js b/Sprint-1/implement/max.test.js
index ff3fbb5e5..f13026fb5 100644
--- a/Sprint-1/implement/max.test.js
+++ b/Sprint-1/implement/max.test.js
@@ -16,7 +16,39 @@ const findMax = require("./max.js");
 // When passed to the max function
 // Then it should return -Infinity
 // Delete this test.todo and replace it with a test.
-test.todo("given an empty array, returns -Infinity");
+test("given an empty array, returns -Infinity", () => {
+  expect(findMax([])).toBe(-Infinity);
+});
+
+test("given one number, returns it", () => {
+  expect(findMax([7])).toBe(7);
+});
+
+test("given positive and negative numbers, returns the largest", () => {
+  expect(findMax([30, -50, 10, 40])).toBe(40);
+});
+
+test("given only negative numbers, returns the closest to zero", () => {
+  expect(findMax([-10, -3, -20])).toBe(-3);
+});
+
+test("given decimals, returns the largest", () => {
+  expect(findMax([1.5, 2.25, 2.2])).toBe(2.25);
+});
+
+test("throws if an element isn't a number", () => {
+  expect(() => findMax(["hey", 10, "hi", 60, 10])).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("throws if not an array", () => {
+  for (const bad of ["hey", 42, undefined, null, {}]) {
+    expect(() => findMax(bad)).toThrow(
+      new Error("findMax requires an array of numbers")
+    );
+  }
+});
 
 // Given an array with one number
 // When passed to the max function
diff --git a/Sprint-1/implement/mean.js b/Sprint-1/implement/mean.js
index 36909ea1f..fb3402ca4 100644
--- a/Sprint-1/implement/mean.js
+++ b/Sprint-1/implement/mean.js
@@ -1,3 +1,15 @@
-function calculateMean(list) {}
+function calculateMean(list) {
+  if (!Array.isArray(list) || list.some((item) => typeof item !== "number")) {
+    throw new Error("calculateMean requires an array of numbers");
+  }
+  if (list.length === 0) {
+    throw new Error("calculateMean requires a non-empty array");
+  }
+  let total = 0;
+  for (const item of list) {
+    total += item;
+  }
+  return total / list.length;
+}
 
 module.exports = calculateMean;
diff --git a/Sprint-1/implement/mean.test.js b/Sprint-1/implement/mean.test.js
index 847b88676..a5981f5b6 100644
--- a/Sprint-1/implement/mean.test.js
+++ b/Sprint-1/implement/mean.test.js
@@ -23,7 +23,38 @@ const calculateMean = require("./mean.js");
 // When passed to calculateMean
 // Then it should return their mean
 // Delete this test.todo and replace it with a test.
-test.todo("given [1, 2, 6], returns 3");
+test("given [1, 2, 6], returns 3", () => {
+  expect(calculateMean([1, 2, 6])).toBe(3);
+});
+
+test("given a single number, returns it", () => {
+  expect(calculateMean([4])).toBe(4);
+});
+
+test("handles negative and decimal numbers", () => {
+  expect(calculateMean([-2, 2, -4, 4])).toBe(0);
+  expect(calculateMean([1.5, 2.5])).toBe(2);
+});
+
+test("throws for an empty array", () => {
+  expect(() => calculateMean([])).toThrow(
+    new Error("calculateMean requires a non-empty array")
+  );
+});
+
+test("throws for non-arrays and no argument", () => {
+  for (const bad of ["banana", 42, null, {}, undefined]) {
+    expect(() => calculateMean(bad)).toThrow(
+      new Error("calculateMean requires an array of numbers")
+    );
+  }
+});
+
+test("throws for arrays with non-numbers", () => {
+  expect(() => calculateMean([1, "2", 3])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
 
 // Given an array with a single number
 // When passed to calculateMean
diff --git a/Sprint-1/implement/sum.js b/Sprint-1/implement/sum.js
index 9062aafe3..d9ea7575a 100644
--- a/Sprint-1/implement/sum.js
+++ b/Sprint-1/implement/sum.js
@@ -1,4 +1,15 @@
 function sum(elements) {
+  if (
+    !Array.isArray(elements) ||
+    elements.some((item) => typeof item !== "number")
+  ) {
+    throw new Error("sum requires an array of numbers");
+  }
+  let total = 0;
+  for (const item of elements) {
+    total += item;
+  }
+  return total;
 }
 
 module.exports = sum;
diff --git a/Sprint-1/implement/sum.test.js b/Sprint-1/implement/sum.test.js
index a288e0f0a..b116cea52 100644
--- a/Sprint-1/implement/sum.test.js
+++ b/Sprint-1/implement/sum.test.js
@@ -13,7 +13,33 @@ const sum = require("./sum.js");
 // Given an empty array
 // When passed to the sum function
 // Then it should return 0
-test.todo("given an empty array, returns 0")
+test("given an empty array, returns 0", () => {
+  expect(sum([])).toBe(0);
+});
+
+test("given one number, returns it", () => {
+  expect(sum([5])).toBe(5);
+});
+
+test("given negative numbers, returns the correct total", () => {
+  expect(sum([10, -30, 5])).toBe(-15);
+});
+
+test("given decimals, returns the correct total", () => {
+  expect(sum([0.5, 0.25, 1])).toBe(1.75);
+});
+
+test("throws if an element isn't a number", () => {
+  expect(() => sum(["hey", 10, "hi", 60, 10])).toThrow(
+    new Error("sum requires an array of numbers")
+  );
+});
+
+test("throws if not an array", () => {
+  for (const bad of ["hey", 42, undefined, null, {}]) {
+    expect(() => sum(bad)).toThrow(new Error("sum requires an array of numbers"));
+  }
+});
 
 // Given an array with just one number
 // When passed to the sum function
diff --git a/Sprint-1/refactor/includes.js b/Sprint-1/refactor/includes.js
index 29dad81f0..8c9ae2e66 100644
--- a/Sprint-1/refactor/includes.js
+++ b/Sprint-1/refactor/includes.js
@@ -1,8 +1,7 @@
 // Refactor the implementation of includes to use a for...of loop
 
 function includes(list, target) {
-  for (let index = 0; index < list.length; index++) {
-    const element = list[index];
+  for (const element of list) {
     if (element === target) {
       return true;
     }
diff --git a/Sprint-1/stretch/aoc-2018-day1/solution.js b/Sprint-1/stretch/aoc-2018-day1/solution.js
index e69de29bb..3435f615a 100644
--- a/Sprint-1/stretch/aoc-2018-day1/solution.js
+++ b/Sprint-1/stretch/aoc-2018-day1/solution.js
@@ -0,0 +1,28 @@
+const fs = require("fs");
+const path = require("path");
+
+const changes = fs
+  .readFileSync(path.join(__dirname, "input.txt"), "utf8")
+  .split(/\r?\n/)
+  .filter((line) => line.trim() !== "")
+  .map(Number);
+
+// Part 1: resulting frequency after all changes
+const finalFrequency = changes.reduce((total, change) => total + change, 0);
+console.log("Part 1:", finalFrequency);
+
+// Part 2: first frequency reached twice
+let frequency = 0;
+const seen = new Set([0]);
+let firstRepeat;
+while (firstRepeat === undefined) {
+  for (const change of changes) {
+    frequency += change;
+    if (seen.has(frequency)) {
+      firstRepeat = frequency;
+      break;
+    }
+    seen.add(frequency);
+  }
+}
+console.log("Part 2:", firstRepeat);