Question 940919
Kate could do 10 jobs in 20 minutes, so she does  10 jobs/20 min or 0.5 job per min. Ethan could do 15 jobs in 35 minutes so he does 15 jobs/35 min or 0.429 job per minute. Working together they would do (0.5+0.429)=0.929 job/min. To do 30 jobs at 0.929 job per minute:
Time=Jobs/rate=30/0.929=32.3 minutes