Question 1146094
In all the examples, 7200 cans are produced.  So, from this point forward, think of producing 7200 cans as the completed job.  (Meaning, in your calculations, you will not have to concern yourself with the number 7200 again.)
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New machine's rate of work = 1/180 job per minute
Older machine's rate of work = 1/360 job per minute
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Rate of work of both machines working together = 1/180 + 1/360 = 2/360 + 1/360 = 3/360 = 1/120 job per minute
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Since both machines working together do 1/120 job per minute, it stands to reason that it takes 120 minutes to complete the job.