There is a formula that estimates how much your puppy will weigh when it reaches adulthood. The method we present applies to medium-sized breeds. First find your puppy's weight w at an age of a weeks, where a is 16 weeks or less. Then the predicted adult weight

W = W(a, w),

in pounds, is given by the formula

W = 52

w

a

.

In this exercise we consider puppies that weigh

w = 4

pounds at age a.

(a) Write a formula for W as a function of the age a.