Question 47402
Here's one approach: Find the hourly rates of each person.

If the painter can do the whole job in 10 hours, then he can do 1/10 of the job in 1 hour.
If his son can do the whole job in 18 hours, then he can do 1/18 of the job in 1 hour.

Together, they can do (1/10 + 1/18) of the job in 1 hour.
So together, in 1 hour they can do (9/90 + 5/90 = 14/90) of the job in 1 hour.

This means it will take them 90/14 hour to do the whole job together.

90/14 hour = 6 hrs and 25.7 minutes.