Two pipes; first fills in $t_1$ min, second empties in $t_2$ min ($t_2 > t_1$). Together fills in—

The net filling rate is $\dfrac{1}{t_1}-\dfrac{1}{t_2}=\dfrac{t_2-t_1}{t_1 t_2}$, so the tank fills in min. The outlet partially offsets the inlet, lengthening the time.