The output of an economic system Q, subject to two inputs, such as labor L and capital K, is often modeled by the Cobb-Douglas production function Q = cL^aK^b. When a + b = 1, the case is called constant returns to scale. Suppose Q = 1280, a = 1/3, b =2/3, and c = 40.
a. Find the rate of change of capital with respect to labor, dK/dL.

b. Evaluate the derivative in part (a) with L = 8 and K = 64.