The answer is 7.5 days
explanation:
80 is four times 20.
80 people should be able to do the job four times faster than 20 people.
So, they should take 1/4 the time.
If 20 people take 30 days, 80 people should take 30/4=7.5 days.
**
simpler explanation:
30*20/80=7.5
