Chapter 1 - Section 1.2 - Formulas and Percent - 1.2 Exercises - Page 61: 5

$r=\dfrac{I}{pt}$

Using the properties of equality, in terms of $ r ,$ the given formula, $ I=prt ,$ is equivalent to \begin{array}{l}\require{cancel} \dfrac{I}{pt}=\dfrac{prt}{pt} \\\\ \dfrac{I}{pt}=r \\\\ r=\dfrac{I}{pt} .\end{array}