diff --git a/src/tax.py b/src/tax.py
--- a/src/tax.py
+++ b/src/tax.py
@@ -6,5 +6,5 @@
 def order_total(prices, discount=0.0, vat_rate=VAT_RATE):
     """Total for an order: prices summed, discount applied, then VAT added."""
     subtotal = sum(prices)
-    discounted = subtotal * (1 + discount)
+    discounted = subtotal * (1 - discount)
     return round(discounted + discounted * vat_rate, 2)