Two pipes X and Y can fill a tank in 14 hours and 21 hours, respectively. Both pipes are opened simultaneously to fill the tank. In how many hours will the empty tank be filled?
8 2⁄5 hours
Work done by pipe X in 1 hour = 1/14
Work done by pipe Y in 1 hour = 1/21
Total work done in 1 hour = (1/14) + (1/21) = (3+2)/42 = 5/42
Time required to fill the tank = 42/5 = 8 2/5 hours
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