Question
Two pipes A and B can fill a tank in 12 hours  andÂ
15 hours  respectively. A third pipe C can empty the full tank in 20 hours . If all three pipes are opened together, in how many hours will the tank be completely filled ?Solution
Rate of A = 1/12 tank per hour Rate of B = 1/15 tank per hour Rate of C = −1/20 tank per hour (emptying) Net rate = 1/12 + 1/15 − 1/20 LCM of 12, 15, 20 = 60 1/12 = 5/60 1/15 = 4/60 1/20 = 3/60 Net rate = (5 + 4 − 3) / 60 = 6 / 60 = 1/10 tank per hour Time to fill the tank = 1 ÷ (1/10) = 10 hours
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