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3-128.

A rodent got into Barndale’s Department Store and chewed right through one of the store displays. Sidney and her manager Eloise need to work together to build a new display. When Sidney works alone she can build the display in hours. When Eloise works alone it takes her hours. How much time will it take to build the new display if Eloise and Sidney work together?   

Sidney's rate is 1 job/7 hours.
Eloise's rate is 1 job/5 hours.

Let t = time to complete the job working together in hours. Then: