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:

Efficiency of X = 140% of Y = 1.4Y

Efficiency of Z = 60% of Y = 0.6Y

Efficiency of X, Y and Z = 1.4 : 1 : 0.6 = 7 : 5 : 3

They can complete the task in 21 days so,

Total work = sum of efficiency ratio $$\times times = (7 + 5 + 3) \times 21 = 315$$

Y and Z together worked for 35 days so,

Work done by Y and Z = (5 + 3) $$\times$$ 35 = 280

Remaining work = 315 - 280 = 35

Remaining work complete by X = work/efficiency = 35/7 = 5 days