Pipe A is capable of filling a tank in 16 minutes while Pipe B can fill the same tank in 24 minutes. Both pipes are opened togeth…

Question

Pipe A is capable of filling a tank in 16 minutes while Pipe B can fill the same tank in 24 minutes. Both pipes are opened together, but after 4 minutes, the rate of Pipe A doubles and the rate of Pipe B becomes half of its original. How many more minutes will it take to fill the tank completely?

1. A. 4 min (Correct answer)
2. B. 5 min
3. C. 6 min
4. D. 7 min

Correct Answer

Option A — 4 min

Detailed Solution & Explanation

The correct answer is 4 min.

Key Points

• Take the tank as LCM(16, 24) = 48 units.
• Then A = 3 units/min, B = 2 units/min.
• In the first 4 minutes they fill (3 + 2)·4 = 20 units, leaving 28.
• Now A doubles to 6 and B halves to 1, so together they do 7 units/min.
• Remaining time = 28 ÷ 7 = 4 minutes.
• The unitary 'LCM as total work' method converts pipe/time problems into simple integer rates and avoids fraction-heavy algebra.