GIVEN: Two pipes can fill a tank in 12 hours and 15 hours respectively. CONCEPT: Total work = Time × Efficiency CALCULATION: Suppose total capacity of tank = 60 units (LCM of 12 and 15) So, Efficiency of pipe 1 = 60/12 = 5 units Efficiency of pipe 2 = 60/15 = 4 units So, Tank filled in 4 hours = (5 + 4) × 4 = 36 units Remaining tank to be filled = 60 – 36 = 24 units Hence, Remaining 24 units of work will be filled by second pipe in 24/4 = 6 hours.