The value of y varies jointly with the values of x and z. When $\displaystyle{x}={4}\usepackage{cmbright}\Huge\$ and $\displaystyle{z}={16}\usepackage{cmbright}\Huge\$ , the value of y is 320. When $\displaystyle{x}={8}\usepackage{cmbright}\Huge\$ and $\displaystyle{z}={10}\usepackage{cmbright}\Huge\$ , the value of y is _____.