A size 6 dress in Country C is size 52 in Country D. A function that converts dress sizes in Country C to those in Country D is f(x) = 2(x + 20). Find a formula for the inverse of the function described.

a. f^-1(x) = (x - 20)/2

b. f^-1(x) = x - 20

c. f^-1(x) = (x/2) - 20

d. f^-1(x) = (x/2) + 20