‘X’ can finish 50% of a job in 8 days. ‘Y’ takes 8 days more than ‘X’ to complete the entire work, and Y is 20% more efficient than ‘Z’. ‘X’ starts the task alone, on 2nd day he is joined by ‘Y’ only, on 3rd day, he is joined by ‘Z’ only, and on 4th day, he again works alone. They follow this same pattern for 9 days. The combined time taken by ‘X’ and ‘Y’ alone to finish the leftover work is how much percent more/less than that by ‘Z’ alone?

Time taken by X to complete the work = 8 / 0.5 = 16 days Time taken by Y = 16 + 8 = 24 days Time taken by Z = 24 × 1.20 = 28.8 days Let total work = 576 units Efficiency of X = 576 / 16 = 36 units/day Efficiency of Y = 576 / 24 = 24 units/day Efficiency of Z = 576 / 28.8 = 20 units/day Work done in 3 days = 36 + 24 + 20 = 80 units Work done in 9 days = 80 × 3 = 240 units Remaining work = 576 – 240 = 336 units Time by X = 336 / 36 = 9.33 days Time by Y = 336 / 24 = 14 days Time by Z = 336 / 20 = 16.8 days Required % = ((9.33 + 14 – 16.8) / 16.8) × 100 = 15% more