A substance, X, is decaying such that the rate of change of the mass, x, is proportional to four times its mass. Another substance, Y, is such that the rate of change of the mass, y, is equal to x - 2y. Initially the values of x and y are 20 and 0, respectively.

a. Find x as a function of t.

b. Find y as a function of t.

c. Determine the maximum value of y, giving your answer in an exact form.