Question 204667
If the man can paint the room in 8 hours, then he can paint {{{1/8}}} of the room in 1 hour.
If his son can paint the room in 12 hours, then he can paint {{{1/12}}} of the room in 1 hour.
Now if the man works by himself for 3 hours, then he can paint:
{{{3*(1/8) = 3/8}}} of the room which leaves {{{5/8}}} of the room to paint. 
Now the man and his son, working together can paint {{{1/8 + 1/12 = 5/24}}} of the room in 1 hour.
So the question is, how long would it take the father & son together to finish the {{{5/8}}} of the room?
To find this, we'll divide the {{{5/8}}} by {{{5/24}}}.
{{{(5/8)/(5/24) = (5/8)*(24/5)}}} = {{{3}}}.
The son works for 3 hours.