Question 1041876
The mason can do 1/10th a wall per hour. Similarly, his apprentice can do 1/15th a wall per hour. So:
1/10+1/15+1/n=1/5
3n+2n+30=6n
n=30
The mason's novice apprentice would take 30 hours  to do the job alone. ☺☺☺☺