The Office Jet printer can copy Janet's dissertation in 20 min. The Laser Jet printer can copy the same documents in 12 min. If the two machines work together, how long would they take to copy the dissertation?
is 1.25 right?
No that is not right. That would be speeding it up about ten times the rate of the fastest printer, just by adding a slow one.
The combined rate is:
1/20 + 1/12 = (6 + 10)/120 = 1/7.5 dissertations per minute.
(1/7.5)*T = 1 dissertation
T = 7.5 minutes