It takes doug 3 days to reroof a house. if doug's son him, the job can be completed in 2 days. how long would it take doug's son, working alone, to do the job?

In the given question, we find that Doug can reroof a house in 3 days working alone. The same job of reroofing takes 2 days to complete if doug's son helps him out.
Firstly we have to determine the amount of roofing doug alone can do in 2 days.
So,
Doug can do 2/3 of the reroofing in 2 days working alone. His son does the rest 1/3 of the roof in 2 days.
So now we have to determine the number of days doug's son will take to do the complete reroofing by himself.
We already know that Doug's son can do 1/3 of the roof in 2 days.
Then the total time required by Doug's son working alone to complete the reroofing will be 6 days. 

It says in the questions that it is 6.5?