diff --git a/README.md b/README.md
new file mode 100644
index 0000000..7c2f62c
--- /dev/null
+++ b/README.md
@@ -0,0 +1,20 @@
+# cart-app
+
+## Cart functions (`src/lib/cart.js`)
+
+- `subtotal(items)` — sum of `price * qty` across all items, before tax.
+- `discount(items, percent)` — total including tax after taking `percent` off the subtotal before tax, rounded to cents.
+- `itemCount(items)` — total quantity across all items.
+- `total(items)` — subtotal plus tax, rounded to cents.
+
+## Running the checks
+
+```
+npm run check
+```
+
+This runs `scripts/check.mjs` and then the test suite. To run only the tests:
+
+```
+npm test
+```
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..b325db5 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -5,6 +5,15 @@ export function subtotal(items) {
   return items.reduce((sum, item) => sum + item.price * item.qty, 0);
 }
 
+export function discount(items, percent) {
+  const discounted = subtotal(items) * (1 - percent / 100);
+  return roundCents(discounted * (1 + TAX_RATE));
+}
+
+export function itemCount(items) {
+  return items.reduce((sum, item) => sum + item.qty, 0);
+}
+
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
diff --git a/tests/lib/cart.test.js b/tests/lib/cart.test.js
index 7a9eab3..6cdcffd 100644
--- a/tests/lib/cart.test.js
+++ b/tests/lib/cart.test.js
@@ -1,6 +1,8 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { subtotal, total } from '../../src/lib/cart.js';
+import { subtotal, total, discount, itemCount } from '../../src/lib/cart.js';
+import { TAX_RATE } from '../../src/generated/rates.js';
+import { roundCents } from '../../src/lib/money.js';
 
 const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
 
@@ -11,3 +13,13 @@ test('subtotal adds price times quantity', () => {
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('itemCount sums quantities across items', () => {
+  assert.equal(itemCount(items), 3);
+  assert.equal(itemCount([]), 0);
+});
+
+test('discount takes percent off the subtotal before tax', () => {
+  assert.equal(discount(items, 10), roundCents(22.5 * (1 + TAX_RATE)));
+  assert.equal(discount(items, 0), total(items));
+});