Question 132053
For work problems the secret is to add the reciprocals of the numbers.
contractor can do 1/20 of the job in 1 hr.
assistant can do 1/30 of the job in 1 hr.
1/20+1/30
=3/60+2/60
=5/60=1/12 of the job working together.
So, the job will take 12 hrs.
.
Ed