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Given

    • A tank can be filled by Pipe1 in 7 hours and by Pipe2 in 5 hours.
    • There is a waste pipe that is kept open when Pipe2 is working; the tank then takes 8 hours 30 minutes to fill.

To Find

• The time taken to fill the tank if all the three pipes are working.


Approach and Working Out

    • Pipe 1 takes 7 hours and pipe 2 takes 5 hours.

    • Pipe 2 and waste pipe takes 8.5 hours.

    • Let us take the total capacity as a common multiple of 5 and 8.5 which is 85.
      o Pipe 2, 17 units/hour
      o Pipe 2 and the waste pipe, 10 units/hour
      o The waste pipe will take out 7 units/hour.

    • Pipe 1 will fill \(\frac{85}{7}\) units per hour.

    • Total input/hour = Pipe 1 + Pipe 2 – Waste pipe.
      o = \(\frac{85}{7} \)+ 17 – 7
      o = \(\frac{155}{7}\)

    • Total time = \(\frac{85}{(155/7) }\)= \(\frac{595}{155}\) = 3.84
      o = 3 hours 50 minutes (approx)


Correct Answer: Option B
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kawal27
A tank can be filled by Pipe1 in 7 hours and by Pipe2 in 5 hours. There is a waste pipe which is kept open when Pipe2 is working; the tank then takes 8 hours 30 minutes to fill. What is the time taken to fill the tank if all the three pipes are working?



A)3 hours 20 minutes

B)3 hours 50 minutes

C)4 hours 15 minutes

D)4 hours 50 minutes

E)5 hours 30 minutes


I would attempt this with reasoning.

Pipe 1 fills in 7 hours. If there were two pipes like 1, they will fill the tank in 3.5 hours (3 hours 30 minute). Since the combination of Pipe 2 & Drain pipe fills at 8.5 hours, I would consider that the answer would have to be just a little bit over 3 hours 30 minutes.

This narrows the answer choices to (B), (C) and may be (D).

Similar logic the other way - If two pipes of the same strength as (pipe 2+ drain pipe) were filling the tank, the tank will be filled in half the time i.e. 4 hours 15 minutes. Since pipe A is faster than this combination, the answer has to be less than 4 hours 15 minutes (which is choice C)

Only (B) remains. I would mark (B) without calculation.
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