John receives utility from consuming X and Y as given by the utility function U(X,Y) = XY. The price of X is $9, and the price of Y is $12.
a. What is John's MRS (marginal rate of substitution)?.
b. What is the optimal mix (ratio) between X and Y in John's market basket?
c. John is currently consuming 15 X and 10 Y per time period. Is he consuming an optimal mix of X and Y? Explain