The carpenter makes 1/4 of the job per hour.
The assistant makes 1/6 of the job per hour.
When they work together, they make 1/4 + 1/6 = 6/24 + 4/24 = 10/24 = 5/12 of the job each hour.
Thus, it will take 12/5 = 2 2/5 hours = 2 hours and 24 minuets for them to finish the job working together.
