A can build a wall in 30 days, while B can destroy it in 40 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(30, 40) = 120 units. Efficiency of A = +4, B = -3. In 2 days, net work = 4 - 3 = 1 unit. We look for when the work reaches 120 - 4 = 116 units. 116 units are done in 116 * 2 = 232 days. On the 233rd day, A works and completes 4 units, making total work 116 + 4 = 120 units. Thus, the wall is completed in 233 days.