Set up a proportion. Cross multiply and use division to find the unknown value.

$\displaystyle\frac{{{1}\ \text{ graphic novel}}}{{{5}\ \text{ books}}}=\frac{{{n}\ \text{ graphic novels}}}{{{50}\ \text{ books}}}\usepackage{cmbright}\Huge\$

$\displaystyle{5}{n}={50}\usepackage{cmbright}\Huge\$

$\displaystyle{n}={10}\usepackage{cmbright}\Huge\$