When you are given a function, you usually have its expression solved for one variable, in terms of the other. In this case, you have an expression for c, written in terms of n:
[tex] c = 20n+15 [/tex]
To compute the inverse, you simply have to solve the expression for the other variable, so in this case we want an expression like
[tex] n = \text{an expression in terms of } c [/tex]
We start by subtracting 15 to both sides:
[tex] c-15 = 20n [/tex]
and then divide both sides by 20 to get the final result:
[tex] n = \cfrac{c-15}{20} [/tex]
