Pipe A can fill a tank in 18 minutes, while pipe B can empty the completely filled tank in 20 minutes. Initially, pipe A is opened and after 6 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: 05 Jun, 2025 Shift 1)

Answer 1

Given:

Pipe A fills the tank in 18 min

Pipe B empties the tank in 20 min

Pipe A is opened alone for 6 min, then B is also opened

Formula used:

Work = LCM of time taken by both Pipes

Efficiency = Work ÷ Time

Calculation:

Total work (LCM of 18 and 20) = 180 units

Efficiency of A = 180 ÷ 18 = 10 units/min

Efficiency of B = - (180 ÷ 20) = -9 units/min

Work done by A in 6 min = 6 × 10 = 60 units

Remaining work = 180 - 60 = 120 units

Combined efficiency of A and B = 10 - 9 = 1 unit/min

⇒ Time to complete 120 units at 1 unit/min = 120 min

∴ The correct answer is \(120\) minutes.

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