GIVEN: Pipe A can fill the cistern in 3 hours and pipe B can fill the cistern in 4 hours. CONCEPT: Total work = Time × Efficiency CALCULATION: Suppose total capacity of cistern = 12 units (LCM of 3 and 4) So, Efficiency of pipe A = 12/3 = 4 units Efficiency of pipe B = 12/4 = 3 units So, A starts first followed by B then by A. So, 4 + 3 + 4 = 11 units of work is completed in 3 hours. Remaining 1 unit of work will be completed by B in 1/3 hours. ∴ Total time = 3 + 1/3 = 10/3 hours