21. Find the discount rate. sweater regular price: $\$ 21$ amount of discount: $\$ 3.15$

Step 1 ：Given that the regular price of the sweater is $21 and the amount of discount is $3.15.

Step 2 ：Calculate the discount rate by dividing the amount of discount by the regular price and then multiplying by 100 to get the percentage.

Step 3 ：\(\text{discount rate} = \frac{\text{discount amount}}{\text{regular price}} \times 100\)

Step 4 ：Substitute the given values into the formula: \(\text{discount rate} = \frac{3.15}{21} \times 100\)

Step 5 ：Simplify the expression to get the discount rate.

Step 6 ：Final Answer: The discount rate is \(\boxed{15\%}\)