posted by kathy .
The OfficeJet printer can copy Maria's dissertation in 18 min. The LaserJet printer can copy the same documents in 14 min. If the two machines work together, how long would they take to copy the dissertation?
Let that time be called T.
1/T = 1/18 + 1/14
1/T = 0.1270
T = 7.88 hours
That is more than half the time of the LaserJet alone.
The reason for the odd number is that you have to add have to the rates of doing the job. You don't average the times and divide by 2.