Let t be the time taken to do the work and m be the number of men employed. Since the time taken to do a piece of work is inversely proportional to the number of men employed, we can write the relationship as t = k/m, where k is the constant of proportionality. From the given information, we know that if it takes 45 men to do a piece of work in 5 days, then 5 = k/45. Solving for k, we find that k = 225.

Now, if 25 men are employed to do the same piece of work, then the time taken would be t = k/m = 225/25 = 9 days. So, it will take 25 men 9 days to do the same piece of work.