12. Chocolate Coupons. A chocolate candy factory places a coupon in each box of candies. For every ten coupons collected, a customer receives a free box of candies. What fraction of the cost of a box does one coupon represent?
12. Common answer: the coupon costs as much as $\frac{1}{10}$ of the chocolate box. This answer is incorrect because the "prize" chocolate box also contains a coupon. Therefore, the customer gets the prize for 9 coupons, not 10. Answer: the cost of the coupon corresponds to $\frac{1}{9}$ of the cost of the chocolate box.