Two pipes P and Q can fill a tank in 10 hours and 14 hours respectively. If both pipes are opened simultaneously, how much time will be taken to fill the tank?
Answer: B
Part filled by P in 1 hour = 1/10
Part filled by Q in 1 hour = 1/14
Part filled by (P + Q) in 1 hour = ( 1/10 + 1/14) = (6/35)
Time taken to fill the tank is (35/6) = 5 hours 49 min