Awall can be built by 6 men working 8 hours per day in 5 days. how many days will it take 4 men to build the wall if they work only 5 hours per day?

Hello,

Let's calculate the number of hours to work for building the wall

1 wall=k*6*8*5=240k
Let's x the number of day to work for the 4 men during 5 hours/day:

1wall=k*4*x*5=20x*k
So 240k=20x*k==>x=240/20=12 (days)