John Consumes Two Goods, X and Y

Essay

John consumes two goods, X and Y. The marginal utility of X and the marginal utility of Y satisfy the following equations:
MU_X = Y MU_Y = X.
The price of X is $9, and the price of Y is $12.
a. Write an expression for John's MRS.
b. What is the optimal mix 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?