Question:
30 men can finish a piece of work in 28 days. How many days will be taken by 21 men to finish it?
Solution:
Let x be the number of days taken by 21 men to finish the piece of work.
More men will take less time to complete the work.
So, this is a case of inverse proportion.
Now, $30 \times 28=21 \times x$
$\Rightarrow x=\frac{30 \times 28}{21}$
$\Rightarrow x=40$
∴ 21 men will take 40 days to finish the piece of work.