First way:

It takes 15 people 8 hours. So it would take 5 people 24 hours.
(That's because 1/3 as many, or 5 people take 3 times as long, or 18 hours.) 

It takes 20 people 6 hours.  (Because 4 times as many people do it
in 1/4th the time.

Answer: 6.

Second way:

Or you can work it:

(people the first time)(hours the first time) = 
   (people the second time)(hours the second time)


15×8 = 20×N

   6 = N

Edwin