How many men required? 2000 men and 500 women can build a bridge in two weeks. If only 1500 men and 250 women are employed, it will take three weeks to build the same bridge. 

How many men will be required if it has to be built in just one week?
[N.B. As Benji Altman points out in the comments to the OP, the question is unclear about the make-up of the work force.  The below assumes that the work force for the question consists solely of men.]

Basic approach. We assume, as is typical in these problems, that each person (possibly differentiated by gender) builds a certain fraction of the bridge per week.  Obviously, this is not truly realistic, but it's the characteristic fiction of this kind of problem.

If we let $m$ and $w$ stand for this fraction, then we have

$$
2000m+500w = \frac{1}{2}
$$

since the combined efforts of $2000$ men and $500$ women suffice to build half a bridge in a week.  Do the same with the other equation, and solve for $m$ and $w$.  Keep in mind that the desired answer—the number of men required to build the bridge in a week—is not $m$ itself, but its reciprocal.  That is to say, if $m = \frac{1}{5000}$ (it isn't; I just chose this number for example), then $5000$ men would be required to build the bridge in one week.



Incidentally, it looks to me as though someone was careful to avoid charges of discrimination.
Every $500$ women there are $2,000$ men for every two weeks. If we need the bridge built in one week then we should increase the amount of men by $150$ percent.