Pipe A (Fill) = 11 hours Pipe B (Empty) = 15 hours ⇒ L.C.M. of 11 and 15 is 165 ⇒ Let the total work be 165 units ⇒ Efficiency of pipe A = 165/11 = 15 units ⇒ Efficiency of pipe B = 165/15 = – 11 units ((Empty)) ⇒ Pipe A and B can fill the tank = 165/(15 – 11) = 165/4 = 41.25 hours ⇒ Pipe A and B can fill the tank = 41 hours 15 minutes ∴ The tank will be filled by pipe A and pipe B in 41 hours 15 minutes.