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.

Their combined rate is $\displaystyle\frac{{1}}{{t}}+\frac{{1}}{{{4}{t}}}=\frac{{4}}{{{4}{t}}}+\frac{{1}}{{{4}{t}}}=\frac{{5}}{{{4}{t}}}\usepackage{cmbright}\Huge\$ posters per hour.

So $\displaystyle\frac{{5}}{{{4}{t}}}\cdot{8}={40}\usepackage{cmbright}\Huge\$ or $\displaystyle\frac{{40}}{{{4}{t}}}=\frac{{40}}{{1}}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle{4}{t}={1}\usepackage{cmbright}\Huge\$ hour.