Pipe A can fill a tank in 9 minutes, while pipe B can empty the completely filled tank in 10 minutes. Initially, pipe A is opened and after 3 minutes pipe B is also opened. In how much time (in minutes) will the remaining tank be filled completely?

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Question

This question was previously asked in

RRB NTPC Graduate Level CBT-I Official Paper (Held On: 06 Jun, 2025 Shift 3)

Answer 1

Given:

Pipe A fills tank in = 9 min

Pipe B empties tank in = 10 min

Pipe A is opened first, and B is opened after 3 min

Calculation:

Assume total capacity of tank = LCM of 9 and 10 = 90 units

Work rate of A = 90 ÷ 9 = 10 units/min (filling)

Work rate of B = 90 ÷ 10 = 9 units/min (emptying)

First 3 minutes: Only A works → Work done = 3 × 10 = 30 units

Remaining = 90 − 30 = 60 units

Now A and B both work together:

Net work per minute = 10 − 9 = 1 unit/min

Time to fill remaining 60 units = 60 ÷ 1 = 60 minutes

Total time to fill the remaining tank = 60 minutes

∴ The correct answer is 60 minutes.

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