Given: Time taken by A to fill tank = 6 hours Time taken by B to fill tank = 8 hours Time taken by A, B and, C together to fill the tank = 12 hours Concept used: Total work = time × efficiency Calculation: Let the capacity of the tank ( work to be done) be 24x units (LCM of 6, 8, 12) ⇒ The efficiency of pipe A = 24x/6 = 4x units/day ⇒ Efficiency of pipe B = 24x/8 = 3x units/day ⇒ Efficiency of pipe (A + B + C) = 24x/12 = 2x units/day ⇒ Efficiency of pipe C = efficiency of (A + B = C) - efficiency of (A + B) Efficiency of pipe C = 2x – (4x + 3x) = – 5x units/day Negative efficiency implies that pipe C is emptying pipe. ⇒ Time taken by pipe C to empty the filled tank = 24x/5x = 4.8 hours or 4 hrs 48 min ∴ The pipe C will empty the tank in 4 hrs 48 mins.