The formula for simple interest is $\displaystyle{I}={P}{r}{t}\usepackage{cmbright}\Huge\$ , where $\displaystyle{P}=\usepackage{cmbright}\Huge\$ principal, $\displaystyle{r}=\usepackage{cmbright}\Huge\$ rate, and $\displaystyle{t}=\usepackage{cmbright}\Huge\$ time. Solve the formula for r.