Izaak Likes to Eat Pizza and to Fish

Izaak likes to eat pizza and to fish. The more fishing he does the happier he is, up to 8 hours a day. If he fishes longer than 8 hours he gets a sore back and is less happy than if he hadn't fished at all. For y less than or equal to 8, his utility function is U(x, y) = x + 3y, where x is money spent on pizza and y is hours per day spent fishing. His income is $40 a day and he has no expenses other than pizza. The Bureau of Fisheries has just decided to allow people without fishing licenses to fish only 3 hours a day. But if you buy a fishing license, you can fish as many hours as you wish. How much is Izaak willing pay for a license?

A) $15
B) $24
C) $18
D) $13
E) $0