10 people can dig a trench in 6 days.
Question:

10 people can dig a trench in 6 days. How many people can dig it in 4 days?

Solution:

Let x people dig the trench in 4 days.

More people will take less number of days to dig the trench. Hence, this is a case of inverse proportion.

Now,$10 \times 6=x \times 4$

$\Rightarrow x=\frac{60}{4}$

$\Rightarrow x=15$

∴ 15 people can dig the trench in 4 days.