Multiple choice

Two inlet pipes A and B can fill an empty tank in 12 and 15 hours respectively, while a drain pipe C can empty full tank in 20 hours. How much time it will take to completely fill the tank if drain pipe was opened after 5 hours?

  1. 10

  2. 7.5

  3. 8

  4. 6.67
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

First 5 hours only A and B are open, filling at rate 1/12 + 1/15 = 3/20 tank per hour, so 5*(3/20) = 3/4 tank fills. Remaining 1/4 tank is filled by all three pipes at rate 1/12 + 1/15 - 1/20 = 1/10 tank per hour, taking (1/4)/(1/10) = 2.5 hours. Total time = 5 + 2.5 = 7.5 hours.

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