Of three pipes, the first two can fill an empty tank in 10.8 hours and 21.6 hours respectively, while the third pipe can empty a full tank in 18 hours. If the tank is empty and all three pipes are opened together, in how many hours will the tank be filled?
12
Take 108 as a convenient common capacity (LCM of 10.8, 21.6 and 18 expressed in tenths).
Rate of first pipe = 108/10.8 = 10 units/hr; rate of second = 108/21.6 = 5 units/hr; rate of outlet = 108/18 = 6 units/hr (negative).
Combined inflow = 10 + 5 - 6 = 9 units/hr.
Time to fill 108 units = 108/9 = 12 hours.
Hence, the tank is filled in 12 hours.
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