X is 25% more efficient than Y. If Y alone can make a chair in 10 days, then X alone can make the chair in how many days?

Let the efficiency of Y be 100 units/day.
Since X is 25% more efficient than Y:

Efficiency of X=100×1.25=125 units/day
The ratio of efficiencies of X and Y is:

Efficiency Ratio (X : Y)=125:100=5:4
Total Work done by Y in 10 days:

Total Work=Efficiency of Y×Time taken by Y=4×10=40 units
Time taken by X alone to finish the same work:

Time=
Efficiency of X
Total Work
​
=
5
40
​
=8 days