Question 136731
Jill does the job in 30 minutes or 1/2 hour.  So turn the fraction over and Jill can do 2/1 or 2 jobs per hour.


Bob does the job in 45 minutes or 3/4 hour.  Bob can do 4/3 job per hour.


Sue does the job in 3/2 hour.  Sue can do 2/3 job per hour.


2 plus 4/3 plus 2/3 = 6/3 + 4/3 + 2/3 = 12/3 = 4 jobs per hour working together.


Turn the fraction (4/1) over again, and together they can do the job once in 1/4 hour or 15 minutes.