Each clock represents 3 hours, so:

Trey worked $\displaystyle{3}\cdot{7}={21}\usepackage{cmbright}\Huge\$ hours

Callie worked $\displaystyle{3}\cdot{6}={18}\usepackage{cmbright}\Huge\$ hours

Will worked $\displaystyle{3}\cdot{5}={15}\usepackage{cmbright}\Huge\$ hours