To complete a certain task, X is 40% more efficient than Y, and Z is 40% less efficient than Y. Working together, they can complete the task in 21 days. Y and Z together worked for 35 days. The remaining work will be completed by X alone in:

Let Y's efficiency = 1 unit. X is 40% more efficient (1.4 units) and Z is 40% less efficient (0.6 units). Combined efficiency = 3 units. Total work = 3 × 21 = 63 units. Y + Z work at 1.6 units for 35 days, completing 56 units. Remaining work = 7 units. X at 1.4 units takes 7/1.4 = 5 days. Options A, B, and D are miscalculations.