Explanation

This is a classic inverse proportion problem:
More men → Less time required.

Given:

• 10 men take 40 days

• 20 men will take ? days

Since the number of men and the number of days are inversely proportional:

$$text{Men}_1 times text{Days}_1 = text{Men}_2 times text{Days}_2$$$$10 times 40 = 20 times x$$ $$400 = 20x Rightarrow x = frac{400}{20} = 20$$