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:
![c = 20n+15](https://img.qammunity.org/2019/formulas/mathematics/middle-school/bx41c3id5juztgcr94d8v20vbg9ttpa2di.png)
To compute the inverse, you simply have to solve the expression for the other variable, so in this case we want an expression like
![n = \text{an expression in terms of } c](https://img.qammunity.org/2019/formulas/mathematics/middle-school/k3jpnoaeux34a27pn92xi35ip2dmloj5l9.png)
We start by subtracting 15 to both sides:
![c-15 = 20n](https://img.qammunity.org/2019/formulas/mathematics/middle-school/pojg4nxmfkte6g07z099yden7nsw2qfjpl.png)
and then divide both sides by 20 to get the final result:
![n = \cfrac{c-15}{20}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/qei9y9b7kvuklqqaz61w4brcajg02wbtds.png)