In the problem, we are given that:
x = number of excellent condition paper backs book that is bought
f = number of fair condition paper backs book that is bought
Also let us say that:
a = price of excellent condition paper backs book = $ 2.50
b = price of fair condition paper backs book = $ 0.50
Therefore the total cost of buying the books can be written as a function of x and f:
total cost = a * x + b * f
Substituting the values of a and b into the expression:
total cost = 2.50 x + 0.50 f