math help???? how do you write an equation?????

Question

a CD costs $10 each. If you buy 4 CD's, you get 1 CD free. Write a equation that represents the cost as the function of number of CD's.

Answer 1

Denote by n the number of CD's. Then the cost is
f(n) = 40[n/5] + 10(n-5[n/5])

Here square brackets stand for the greatest integer function, i.e. [x] is the greatest integer smaller than or equal to x.

Answer 2

C = 10x - int(x/5)
where int(x/5) is the integer value of x/5