Very simple. divide 3952 by 4, because the larger fish is 3 times the amount of the small fish, so there are 4 groups. Multiply it by 3 for the larger fish and keep it for the small fish. Then, divide the products by 13 because the quotient is the amount of money each fish costs. There you have your answer!
She'll need to use 14 , 1/4 cups to make 3 1/2 cups