Charlie likes both bananas and apples. He consumes nothing else. Charlie consumes x1 bushels of apples per year and x2 bushels of bananas per year. Assume that Charlie's preference is represented in the following utility function: (x1 x2) = x1 x2 . Also, assume that the price of x1 is $1, the price of x2 is $2, and Charlie's income is $40. Determine the function of budget constraint for Charlie and the marginal utility for the apples and bananas?