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CAT 2020 Slot 2 · Quant · Pipes and Cisterns · easy

Pipes A and B can fill a tank in 10 hours and 15 hours respectively, while pipe C can empty the full tank in 12 hours. If all three pipes are opened…

Single correct option

Pipes A and B can fill a tank in 10 hours and 15 hours respectively, while pipe C can empty the full tank in 12 hours. If all three pipes are opened together when the tank is empty, how long will it take to fill the tank?
  1. A. 10 hours
  2. B. 12 hours
  3. C. 15 hours
  4. D. 6 hours
Show answer and solution

Answer: (B) 12 hours

Take the tank as 60 units (LCM of 10, 15, 12).

A fills 6 units/h, B fills 4 units/h, C empties 5 units/h. Net = 6 + 4 − 5 = 5 units/h.

Time = 60/5 = 12 hours.

Tip: the LCM-as-total-work method avoids fractions entirely.
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Original question written in the CAT 2020 Slot 2 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake