diff --git a/src/format.ts b/src/format.ts
index 0115f48..a2d9c35 100644
--- a/src/format.ts
+++ b/src/format.ts
@@ -1,7 +1,17 @@
-export function formatCurrency(amount: number) {
-  return `$${amount.toFixed(2)}`;
+export function formatCurrency(amount: number, currency = "USD") {
+  return new Intl.NumberFormat("en-US", {
+    style: "currency",
+    currency,
+  }).format(amount);
 }
 
 export function formatPercent(value: number) {
   return `${(value * 100).toFixed(1)}%`;
 }
+
+export function formatNumber(value: number, decimals = 2) {
+  return new Intl.NumberFormat("en-US", {
+    minimumFractionDigits: decimals,
+    maximumFractionDigits: decimals,
+  }).format(value);
+}
diff --git a/src/math.ts b/src/math.ts
index a942c9a..0786b9c 100644
--- a/src/math.ts
+++ b/src/math.ts
@@ -3,13 +3,16 @@ export function add(a: number, b: number) {
 }
 
 export function subtract(a: number, b: number) {
-  return a + b; // bug: should be a - b
+  return a - b;
 }
 
 export function multiply(a: number, b: number) {
-  return a * b + 1; // bug: off by one
+  return a * b;
 }
 
 export function divide(a: number, b: number) {
+  if (b === 0) {
+    throw new Error("division by zero");
+  }
   return a / b;
 }
diff --git a/src/old-module.ts b/src/old-module.ts
deleted file mode 100644
index cf92d8f..0000000
--- a/src/old-module.ts
+++ /dev/null
@@ -1,4 +0,0 @@
-// this module is deprecated and will be removed
-export function legacyHelper() {
-  return "old";
-}
diff --git a/src/validate.ts b/src/validate.ts
new file mode 100644
index 0000000..a5bfb8a
--- /dev/null
+++ b/src/validate.ts
@@ -0,0 +1,11 @@
+export function isPositive(n: number) {
+  return n > 0;
+}
+
+export function isInRange(value: number, min: number, max: number) {
+  return value >= min && value <= max;
+}
+
+export function isInteger(n: number) {
+  return Number.isInteger(n);
+}
diff --git a/test/math.test.ts b/test/math.test.ts
index 67de508..5815895 100644
--- a/test/math.test.ts
+++ b/test/math.test.ts
@@ -17,4 +17,8 @@ describe("math", () => {
   it("divides", () => {
     expect(divide(10, 2)).toBe(5);
   });
+
+  it("throws on division by zero", () => {
+    expect(() => divide(1, 0)).toThrow("division by zero");
+  });
 });