One tap can fill a water tank in 3 hours and another tap can empty it in 4 hours. If the tank is one third full and both the taps are opened together, then the time taken by taps to fill the tank will be-

Explanation

Let the capacity of the tank is 12 liters. 1/3 capacity of tank = $12 \times \frac{1}{3} = 4$ liter. Remaining capacity of tank = $12 - 4 = 8$ liter. Tank filled by first tap in 1 hr. = $\frac{12}{3} = 4$ liter. and Second tap will empty in 1 hr. = $\frac{12}{4} = 3$ liter. Both tap will fill the tank in 1 hr. = $4 - 3 = 1$ liter. Time taken by taps to fill remaining 8 ltr. = $\frac{8}{1} = 8$ hr.