Length $\displaystyle={l}\usepackage{cmbright}\Huge\$ and Width $\displaystyle={l}-{15}\usepackage{cmbright}\Huge\$

$\displaystyle{100}={l}{\left({l}-{15}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{100}={{l}}^{{2}}-{15}{l}\usepackage{cmbright}\Huge\$

$\displaystyle{0}={{l}}^{{2}}-{15}{l}-{100}\usepackage{cmbright}\Huge\$

$\displaystyle{0}={\left({l}-{20}\right)}{\left({l}+{5}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{l}={20}\usepackage{cmbright}\Huge\$ or $\displaystyle{l}=-{5}\usepackage{cmbright}\Huge\$

Since the length is 20, the width is $\displaystyle{20}-{15}={5}\usepackage{cmbright}\Huge\$ .