Which equation below represents the inverse function

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:

To compute the inverse, you simply have to solve the expression for the other variable, so in this case we want an expression like

We start by subtracting 15 to both sides:

and then divide both sides by 20 to get the final result:

answer: x=60