Pipes A and B are filling pipes while pipe C is an emptying pipe. A and B can fill a tank in 72 and 90 min respectively. When all the three pipes are opened together, the tank gets filled in 2 hrs. A and B are opened together for 12 minutes, then closed and C is opened. The tank will be empty after ?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 18 minutes.
A: 72 → 5
B: 90 → 4 LCM 360
A + B work in 2 hrs = (5 + 4) 120
= 1080
But the capacity of tank is 360
= 1080 - 720 = 360
(720 = work done by C)
= 720/120 = 6 units/minute
(A + B)'s 12 min work = 9 × 12 = 108
C's time to complete 108 work = 108/6
= 18 min