Question

15 men can do a piece of work in 36 hours how many men will be needed to finish the work in 20 hours

Answer 1

It will require 48 men that can do it in 20 hours. 

Answer 2

Men is inversely proportional to Time && directly proportional to Work
M1 * Hours = M2 * Hours

15*36 = M2 * 20
M2 = 15*36/20 = 27 men

OR
15 men--36 hours=== 1 work
1 man----1 hour == 1/(36*15) work
1 man...20 hours == 20/(36*15) work
1 man==1/27 work
X men== 1 work
X/27 = 1 
X = 27 men.
Hope it helps.