Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the logic errors pattern
The wrong operator, an inverted condition, a formula the right way round but backwards.
What went wrong: apply_discount does 200 - 50. A 50% discount is 200 - (200 × 50 / 100).
The fix: in pricing.py, return total - total * pct / 100
Remember: the file where you see the wrong answer is not always the file with the bug. Follow the function call.
The pattern behind it: logic errors.