Question 999272
Al takes 4 hours to complete the job
so he does 1/4 of the job in hour

He worked 1.5 hours earlier
so he completed 1/4 * 1.5 = 3/8 of the job before Bob started

Balance job = 1-3/8 = 5/8 

Together they doe 7/12 of the job in 1 hour

So to complete 5/8 of the job they take 

{{{(5/8)/(7/12)}}} hours

= 5/8 * 12/7 = 15/14 hours 

15/14 + 3/2 = 36/14 = 18/7 hours