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One pipe can fill the tank in 21 minutes while another pipe can empty completely filled tank in 24 minutes. If both the pipes are operated together on empty tank, how long (in minutes) will it take to fill one-fourth of the tank?

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RRB NTPC Graduate Level CBT-I Official Paper (Held On: 06 Jun, 2025 Shift 1)
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  1. 126
  2. 84
  3. 168
  4. 42

Answer (Detailed Solution Below)

Option 4 : 42
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Detailed Solution

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Given:

Filling pipe time = 21 min

Emptying pipe time = 24 min

Target = 1/4 of tank

Calculation:

LCM method: Assume tank capacity = LCM of 21 and 24 = 168 units

Filling rate = 168 ÷ 21 = 8 units/min

Emptying rate = 168 ÷ 24 = 7 units/min

Net rate = 8 − 7 = 1 unit/min

⇒ 1/4 of 168 = 168 ÷ 4 = 42 units

⇒ Time = 42 ÷ 1 = 42 min

∴ Time to fill 1/4 tank = 42 minutes

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