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 minutes respectively. When all the three pipes are opened together, the tank gets filled in 2 hours. A and B are opened together for 12 minutes, then closed and C is opened. The tank will be empty after:

Answer: Option C

Solution

Answer: Option C

Solution:

A + B - C = 3
5 + 4 - C = 3
C = 6
In 12 min, (A + B) can fill = (5 + 4) × 12 = 9 × 12
The time was taken to empty (9 × 12) by 'C'
Time $$=\frac{9\times 12}{6}=18\min$$