If 35 men can reap a field in 8 days,

Let x be the required number of days. Then, we have:

Clearly, less men will take more days to reap the field.

So, it is a case of inverse proportion.

Now, $8 \times 35=x \times 20$

$\Rightarrow \frac{8 \times 35}{20}=x$

$\Rightarrow 14=x$

Therefore, 20 men can reap the same field in 14 days.