The time taken by ‘A’ to build a wall alone is 20%

Solution

Let the work ‘A’ and ‘B’, can do in 1 day be ‘a’ units and ‘b’ units, respectively. Let they takes ‘p’ days to build the wall, together Therefore, Total work = (a + b) × p units …….. (1) According to the question, Time taken by ‘A’ to build the wall alone = 1.20p days Therefore, total work = 1.20p × a units….. (2) From equation (1) and (2), we get 5b = a Therefore, total work to be done = (a + b) × p = 6bp units Time taken by 'B' to build the wall = 6bp/b = 6p days Therefore, required percentage = (6p – p)/p} × 100 = 500% more time