A can build a wall in 15 days, and B can destroy it in 20 days. If they work on alternate days starting with A, in how many days will the wall be built? MCQ with Answer and Explanation

Total work = LCM(15, 20) = 60 units. Efficiency of A = +4, B = -3. In 2 days (A then B), net work done = 4 - 3 = 1 unit. We need to reach 60 - 4 = 56 units first. 56 units will be done in 56 * 2 = 112 days. On the 113th day, A works and completes 4 units, making the total work 56 + 4 = 60 units. So, the wall is completed in 113 days.