Chapter 1 - Section 1.2 - Formulas and Percent - 1.2 Exercises - Page 62: 22

$r=\dfrac{M-C}{C}$

Using the properties of equality, in terms of $ r ,$ the given formula, $ M=C(1+r) ,$ is equivalent to \begin{array}{l}\require{cancel} M=C+Cr \\\\ M-C=Cr \\\\ \dfrac{M-C}{C}=r \\\\ r=\dfrac{M-C}{C} .\end{array}