working together, could do a job in 1 hour and 20 minutes  = 80 minutes

After Shirley has worked for 2 hours, Julia joins her. They work together and finish the jod in 40 minutes 

it means that Shirley  finished half of the work alone use 2 hours

so Shirley could finish the job in 4 hours woring alone

4 hours = 240 minutes

it means that Shirley could do 1/240 of the job in one minutes

so 40 minutes Shirley can do 1/6 of the job

they use 40minutes finish half of the job so the left job all do by Julia

that means Julia finish 2/6 of the job in 40 minutes

so Julia finish all job alone in 120 minutes = 2hours