If 24 Men can do a work in 35 days, then the number of men required to complete the same work in 21days is

24 men can do a work in 1 day $=\frac{1}{35}$

1 man can do a work1 day $=\frac{1}{35\times 24}$

Number of men required to complete work in 21 days

$=1\text{\hspace{0.17em}day work}\times 21\text{\hspace{0.17em}days}$

$\text{=}\frac{1}{35\times 24}\times 21=\frac{1}{40}$

40 men are required.