1. **State the problem:** A customer buys shoes for 39.99, a shirt for 19.99, and a belt for 9.99. They pay with 100. We need to find the change. 2. **Formula:** Change = Amount given - Total cost 3. **Calculate total cost:** $$39.99 + 19.99 + 9.99 = 69.97$$ 4. **Calculate change:** $$100 - 69.97 = 30.03$$ 5. **Rounding check:** Rounded prices are 40 + 20 + 10 = 70, so change approx 100 - 70 = 30, close to exact 30.03. **Final answer:** The customer receives $30.03 in change.