In one hour, 1/6 of the tank will be filled and 1/10 of the tank will be emptied. As a net result,
[tex]\begin{aligned}\dfrac{1}{6} - \dfrac{1}{10} &= \dfrac{1}{6}\cdot\bold{\dfrac{5}{5}} - \dfrac{1}{10}\cdot\bold{\dfrac{3}{3}}\\[0.5em]&= \dfrac{5}{30} - \dfrac{3}{30}\\[0.5em]& = \dfrac{2}{30}\\[0.5em]& = \dfrac{1}{15}\end{aligned}[/tex]
we see that 1/15 of the tank is filled per hour when both valves are open.
It will take 15 hours to fill the tank.
