You probably set up a rate incorrectly. The first potter's rate is $\displaystyle\frac{{15}}{{5}}\usepackage{cmbright}\Huge\$ or 3 mugs per hour. The second potter's rate is $\displaystyle\frac{{12}}{{3}}\usepackage{cmbright}\Huge\$ or 4 mugs per hour. Find their combined rate. Find a fraction that represents the rate for the new job in mugs per hour. Set these two numbers equal and solve the equation. The correct answer is 9 hours.