30 men can finish a piece of work in 28 days.

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.