A pipe can fill a tank in 12 hours. Another pipe can empty the filled tank in 28 hours. If both the pipes are opened simultaneously, then the time (in hours) in which the tank will be one-third filled, is:

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RRB Technician Grade III Official Paper (Held On: 29 Dec, 2024 Shift 1)
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  1. 21
  2. 28
  3. 14
  4. 7

Answer (Detailed Solution Below)

Option 4 : 7
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Detailed Solution

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Given:

Pipe 1 fills the tank in 12 hours.

Pipe 2 empties the tank in 28 hours.

Formula used:

Work done = LCM of time taken

Efficiency = Total work / Time

Calculation:

LCM of 12 and 28 = 84

Let the total capacity of the tank be 84 units.

Efficiency of Pipe 1 (filling) = 84 / 12 = 7 units/hour

Efficiency of Pipe 2 (emptying) = 84 / 28 = 3 units/hour

Net efficiency when both pipes are open = 7 - 3 = 4 units/hour

One-third capacity of the tank = 84 / 3 = 28 units

Time taken to fill one-third of the tank = 28 / 4 = 7 hours

∴ The time taken to fill one-third of the tank is 7 hours.

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