Question 1079215
The answer above and the answers below are all correct.
I like AnlytcPhil's method below best.  

It takes 12 workers which is 7 more than the original 5

1 worker takes 240 days
5 workers take 240/5 = 48 days
6 workers take 240/6 = 40 days, which is 1 more worker and
8 days less, which checks.

Thus 12 workers takes 240/12 = 20 days, which is 28 days less
than the 48 days it takes 5 workers, which checks.

Edwin