If X works 3 tims as fast as Y and is able to complete, a work in 40 days less than the number of days taken by Y, then find the time in which they cn complete the work working together.

Explanation

If Y does the work in 3 days, X does it in 1 day and Y takes 3 days. If different is 40 days. But the actual difference is 40 days
If difference is 3 days, X takes 1 day and Y takes 3 days. If difference is 40 dayas (i.e., 20 times), X takes 20 days and Y 60 days.
`:.` Take taken together =`(20xx60)/(20+60)= 15 days