Given:
10 boys clear a piece of land in 21 days.
To find:
The number of days it will take 7 boys to clear the same land at the same rate.
Solution:
We have,
10 boys clear a piece of land in 21 days.
Total work =

=

Let 7 boys clear the same piece of land in x days.
Total work =

=

Now, the total work is same.

Divide both sides by 7.


Therefore, 7 boys clear the same piece of land in 30 days.