To solve this we should focus on the fact that here the speed with which each person works is constant throughout.

Also it's convenient if we know ( work = work speed × time )

Speed of 6 men = ( 1 work / ( 6(hrs/day)×10 days )) = (1/60)

Speed of 1 man = 1/360

Speed of X men = ( 2 work / ( 5(hrs/day)× 4 days )) = 1/10

Speed of 1 man = 1/(10*X) = 1 / 360

Therefore X = 36 men which is **30** more men from the first case

So we need **30** more men to complete double the work in shorter time frame of 20 hrs vs 60 hrs.