You may want to rewrite the product first: $\displaystyle{I}={P}{t}{r}\usepackage{cmbright}\Huge\$

Divide both sides by $\displaystyle{P}{t}\usepackage{cmbright}\Huge\$ : $\displaystyle\frac{{I}}{{{P}{t}}}=\frac{{{P}{t}{r}}}{{{P}{t}}}\usepackage{cmbright}\Huge\$

Simplify to: $\displaystyle\frac{{I}}{{{P}{t}}}={r}\usepackage{cmbright}\Huge\$