Correct. Using the equation for work, $\displaystyle{r}=\frac{{W}}{{t}}\usepackage{cmbright}\Huge\$ , you find that the rate of the fast pipe is $\displaystyle\frac{{1}}{{p}}\usepackage{cmbright}\Huge\$ (because it fills 1 pool in p hours), and the rate of the slow pipe is $\displaystyle\frac{{1}}{{{2}{p}}}\usepackage{cmbright}\Huge\$ (because it takes twice as long for the slow pipe to do the same amount of work as the fast pipe). Since you know that it takes both pipes 5 hours to fill 1 pool, you can write $\displaystyle\frac{{1}}{{p}}+\frac{{1}}{{{2}{p}}}=\frac{{1}}{{5}}\usepackage{cmbright}\Huge\$ . Solving for p yields p=7.5, so 2p (the time of the slow pipe) = 2(7.5) = 15 hours.