It was calculated that 75 men could complete a piece of with Answer

It was calculated that 75 men could complete a piece of work in 20 days. When work was scheduled to commence, it was found necessary to send 25 men to another project. How much longer will it take to complete the work?

Answer:
30 days.
 
Explanation:
Before:

One day work = 1 / 20
One man’s one day work = 1 / ( 20 * 75)

Now:

No. Of workers = 50
One day work = 50 * 1 / ( 20 * 75)

The total no. of days required to complete the work = (75 * 20) / 50 = 30