To rewrite any equation, you must follow the same rules that you learned earlier. For example, multiply or divide both sides by the same number. To isolate r in the original equation, you need to divide both sides by $\displaystyle{P}{t}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{r}=\frac{{I}}{{{P}{t}}}\usepackage{cmbright}\Huge\$ .