Given: A can fill the tank = 4 hours B can empty the tank = 6 hours Concept used: Total capacity = LCM Filled tank in unit value of time is known as efficiency The efficiency of inlet pipes is positive The efficiency of outlet pipes is negative Formula used: Efficiency = total capacity/total time Calculations: Let the total capacity of the tank be x The efficiency of A = x/4 The efficiency of B = -x/6 The total efficiency of A and B together = (x/4) – (x/6) ⇒ (3x – 2x)/12 ⇒ x/12 Total time is taken by A and B to fill the whole tank = x/(x/12) ⇒ 12 hours ∴ Total time is taken by A and B to fill the whole tank = 12 hours