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part1-pricing/README.md

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+# Part 1 — Pricing: discount stacking
+
+`pricing/discounts.py` contains `apply_discounts()`, used by our billing
+service to price orders that carry one or more percentage discounts. The
+current implementation does not follow the billing rules below, and part of
+the test suite fails because of it.
+
+## Your task
+
+Update `apply_discounts()` so it implements the billing rules. Run the tests
+with:
+
+```bash
+pip install -e ".[dev]"
+python -m pytest
+```
+
+## Billing rules (specification)
+
+1. `base_price` is a non-negative number. A negative `base_price` raises
+   `ValueError`.
+2. `discounts` is a sequence of percentages. Each discount must be within
+   `0`–`100` inclusive; any discount outside that range raises `ValueError`.
+3. **Stacking is sequential (compounding).** Each discount applies to the
+   price left over by the previous one — discounts are never summed.
+
+   Example: `100.00` with discounts `[10, 20]`:
+   - after 10%: `90.00`
+   - after 20% of the remainder: `72.00` ← final price
+
+4. The result is rounded to 2 decimal places.
+
+That is the whole specification. When you are done, note anything worth
+mentioning in the repository-level `NOTES.md`.