You probably forgot to take into account the initial fee of $50, and divided the $150 by $8. If $\displaystyle{h}=\usepackage{cmbright}\Huge\$ the number of hours, then Esmeralda is charged $\displaystyle{8}{h}\usepackage{cmbright}\Huge\$ dollars plus the $50. So $\displaystyle{8}{h}+{50}={150}\usepackage{cmbright}\Huge\$ .