the officejet printer can copy Janet's dissertation in 10 minutes. The laserjet printer can print the same copy in 20 minutes. If the two machines work together, how long would they take to copy the dissertation?

Answer 1

It would take 10 minutes.

Answer 2

rate office jet = 1copy/10 min
rate of laserjet = 1copy/20 min

combined rate = 1copy/10 + 1copy/20 = 3copy/20minutes

time for combined rate = 1 copy/(3copy/20min) = 20/3 minutes or
6 minutes, 40 seconds