Correct.

$\displaystyle{y}={k}{x}{z}\usepackage{cmbright}\Huge\$ , so $\displaystyle{320}={k}{\left({4}\right)}{\left({16}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{320}={64}{k}\usepackage{cmbright}\Huge\$ , so $\displaystyle{k}={5}\usepackage{cmbright}\Huge\$

$\displaystyle{y}={k}{x}{z}\usepackage{cmbright}\Huge\$ , so $\displaystyle{y}={5}{\left({8}\right)}{\left({10}\right)}\usepackage{cmbright}\Huge\$ , so $\displaystyle{y}={400}\usepackage{cmbright}\Huge\$