By studying, Will can produce a higher grade, Gw, on an upcoming economics exam. His production function depends on the number of hours he studies marginal analysis problems, A, and the number of hours he studies supply and demand problems, R. Specifically, Gw = 2.5A^(0.36)R^(0.64). His roommate David's grade production function is Gd = 2.5A^(0.25)R^(0.75).

a) What is Will's marginal productivity from studying supply and demand problems? What is David's?

b) What is Will's marginal rate of technical substitution between studying the two types of problems? What is David's?