Question 27
At a fun fair, coupons can be used to purchase food. Each coupon is worth 5, 8 or 12 dollars. For example, for a 15-dollar purchase you can use three coupons of 5 dollars, or use one coupon of 5 dollars and one coupon of 8 dollars and pay 2 dollars by cash. Suppose the prices in the fun fair are all whole dollars. What is the largest amount that you cannot purchase using only coupons?