feat: add percentage discount calculation (#3)

src/demo_toolkit/numbers.py

@@ -7,3 +7,9 @@ def clamp(value: float, minimum: float, maximum: float) -> float:
     if minimum > maximum:
         raise ValueError("minimum cannot be greater than maximum")
     return min(maximum, max(minimum, value))
+
+def calculate_discount(price: float, discount_percent: float) -> float:
+    """Calculate the final price after applying a percentage discount."""
+    if discount_percent < 0 or discount_percent > 100:
+        raise ValueError("Discount percent must be between 0 and 100.")
+    return price * (1 - discount_percent / 100.0)

tests/test_numbers.py

@@ -1,6 +1,7 @@
 import pytest
 
 from demo_toolkit.numbers import clamp
+from demo_toolkit.numbers import calculate_discount
 
 
 def test_clamp_limits_both_sides():
@@ -11,3 +12,7 @@ def test_clamp_limits_both_sides():
 def test_clamp_rejects_reversed_range():
     with pytest.raises(ValueError, match="minimum"):
         clamp(1, 10, 0)
+
+def test_calculate_discount_applies_percentage():
+    assert calculate_discount(100.0, 20.0) == 80.0
+    assert calculate_discount(50.0, 10.0) == 45.0