math
posted by nono .
the officejet printer can copy lisa's dissertation in 18 min. the laserjet printer can copy the same document in 20 min. if the two machines work together, how long would the take to copy the dissertation

For every minute, the officejet does 1/18 of the work, while the laserjet does 1/20 of the work.
Together the two printers do (1/18+1/20)=19/180 of the work every minute.
If the two work together, and assuming the work can be shared to keep the two busy all the time, the work can be done in 1/(19/180) = 180/19 = 9.47 minutes. 
time = 1/(1/18 + 1/20)
= 1/(19/180) = 180/19 = 9.47 minutes
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