Math
posted by Sue on .
The officejet printer can copy Sue's dissertation in 18 min. The laserjet printer can copy the same document in 22 min. If the two machines work together how long would they take to copy the dissertation?
Any help would be greatly appreciated.

In 18*22min, laserjet can do 22, and the officejet can do 18. So the rate is
rate= number/time= (18+22)/22*18=40/396min = 1dissertation/9.9 min
that answers it, but formally
dissertations= rate*time
time= dissertations/rate= 1dissertation/(1dissertation/9.9min)= 9.9 min 
Would it be 99/10?

Thank you for your help.

yes, but few folks say 99/10 min, most would say 9.9 minutes, or 9 minutes 54seconds.