Rupert is a cattle farmer. He invests all his spare cash in buying calves and raising them on otherwise useless captured land behind his house. The cows double in weight during the first year, after which time they are sold. Cows are bought and sold at a constant price per pound. Rupert's friend, Wally, wants a loan from Rupert. What is the interest rate Wally have to pay for Rupert to recover his opportunity cost of making the loan?