Set up a proportion and use cross products to solve.

$\displaystyle\frac{{4}}{{n}}=\frac{{24}}{{36}}\usepackage{cmbright}\Huge\$

$\displaystyle{24}{n}={144}\usepackage{cmbright}\Huge\$

$\displaystyle{n}={6}\usepackage{cmbright}\Huge\$