Two pipes A and B can fill a tank in 18 and 24 minutes respectively. After both are opened together for 6 minutes, the rate of pipe A becomes half, and pipe B becomes double its original. In how many more minutes will the tank be full?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 3 minutes 45 seconds.
Rate of Pipe A = 1/18
Rate of Pipe B = 1/24
Combined rate of Pipes A and B in 1 minute = 1/18 + 1/24 = 7/72
Filled in 6 min = 6 × 7/72 = 7/12
Remaining part = 5/12
New rate of Pipe A = 1/18 × 1/2 = 1/36
New rate of Pipe B = 1/24 × 2 = 1/12
New combined rate of Pipes A and B in 1 minute = 1/36 + 1/12 = 1/9
Time required for remaining part = (5/12) ÷ (1/9)
= (5/12) × 9
= 15/4 minutes
= 3 minutes 45 seconds