You probably set up your equation incorrectly. Let $\displaystyle{t}=\usepackage{cmbright}\Huge\$ the time, in hours, it takes the owner to frame one poster. Then $\displaystyle{4}{t}=\usepackage{cmbright}\Huge\$ the amount of time it takes the apprentice. Their rates are $\displaystyle\frac{{1}}{{t}}\usepackage{cmbright}\Huge\$ and $\displaystyle\frac{{1}}{{{4}{t}}}\usepackage{cmbright}\Huge\$ posters per hour. Add these rates to get the combined rate of the owner and her apprentice. Then use rate times time equals amount of work done.