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Robby Robot can make a widget in 7 minutes. With the help of Roger Robot, the job can be done in 4 minutes. How long would it take Roger Robot working alone?

Let the number of minutes for Roger Robot to complete the job.

Robby Robot will make  of a widget per minute and Robby will make  of a widget per minute.

Write and solve an equation using the format work rate · time = 1 job

They are working together, so the total work rate is