A water tank has two taps ( Tap-1 and Tap-2) : Tap-1 can fill a tank in 8 hours and Tap-2 can empty the tank in 16 hours . How long will they take to fill the tank if both taps are opened simultaneously but Tap -2 is closed after 8 hours?

Correct Answer: 12
In 1 hr, the tank is filled = 1/8 - 1/16 = 1/16 In 8 hrs " " " " = 8/16 = 1/2 Now, 1/8 part is filled by tap - 1 in 1 hour 1/2 part is filled by tap - 1 in = 8/2 = 4 hrs So, the required time = 8 + 4 = 12 hrs