Question 330519
Jack takes 5 hours
so he can do 1/5 of the job in 1 hour
,,
Mary can do it in 4 hours
so she does 1/4 of the job in 1 hour
..
Together they will do 1/5  + 1/4 of the job in 1 hour.
LCD = 20
4+5  / 20
=9/20 of the job in 1 hour
..
they will take 20 / 9 hours to do the job
=2.22 hours