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.