Question 321294
Dixie can paint a room in 3 hours working alone. 
So she can do 1/3 of the job in 1 hour.
Trixie can paint the same room in 6 hours
So she can do 1/6 of the job in 1 hour.
..
Trixie joins 1 hour late
..
Dixie has done 1/3 of the job
When Trixie joins only 1-1/3 of the job is remaining
= 2/3 of the job.
..

Together they do 1/3 +1/6 of the job in 1 hour.
=2+1/ 6
=1/2 the job in 1 hour.
..
1/2 job they together do in 1 hour
2/3 of job they will do in 2/3 / 1/2
=2/3 * 2
1 1/3 hours
They will finish the job in 1 1/3 hours.