Q. Pipe P and Q can fill a tank in 10 and 12 hours respectively and C can empty it in 6 hours. If all the three are opened at 7 am, at what time will one-fourth of the tank be filled?
Answer: 10 pm
Notes: Part of tank filled in 1 hour when all three pipes are opened $latex = \frac{1}{10}+\frac{1}{12}-\frac{1}{6}&s=1$ $latex = \frac{6+5-10}{60} = \frac{1}{60}&s=1$ ∴ The tank will be filled in 60 hours. ∴ One fourth of the tank will be filled in 15 hours so the tank will be filled at 10 pm. Hence option [C] is correct answer.

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