1. **State the problem:** We need to find the sales tax percentage when a chess board priced at 21.70 costs 23.87 after tax. 2. **Formula:** Sales tax percentage is calculated by $ \text{Tax Percentage} = \frac{\text{Tax Amount}}{\text{Original Price}} \times 100\% $. 3. **Calculate the tax amount:** $$\text{Tax Amount} = 23.87 - 21.70 = 2.17$$ 4. **Calculate the tax percentage:** $$\text{Tax Percentage} = \frac{2.17}{21.70} \times 100\%$$ 5. **Simplify the fraction:** $$\text{Tax Percentage} = \frac{\cancel{2.17}}{\cancel{21.70}} \times 100\% = 0.1 \times 100\%$$ 6. **Final calculation:** $$0.1 \times 100\% = 10\%$$ **Answer:** The sales tax percentage is 10%.