Mr. Vella can build the wall in 4 days, however only works on it for 3 days. That means that he has completed 75% of the and is leaving 25% for his apprentice.
We need to figure out how long it takes the apprentice to build 25% of a wall. We know he can build 100% of a wall in 6 days, so dividing 6 days by 4 will give us our answer.
6/4 = 3/2 = 1.5 days
It took the apprentice 1.5 days to finish the wall.
