You may have tried to find the combined rate of the potters by adding the number of mugs and adding the times and then dividing. 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. You need to add these to find their combined rate. The correct answer is 9 hours.