Given the following production function
Q = F(K,L) = K^3/4 L^1/4
a) Find an expression for the marginal product of labor, MPL, when the amount of capital is fixed at 16 units.
b) Assume capital is fixed at 16 units. If the firm can sell its output at price of $100 per unit and can hire labor at $25 per unit, how many units of labor should the firm hire in order to maximize profits?