Two grandparents want to pick up the mess that their granddaughter had made in her playroom. one can do it in 15 minutes working alone. the other, working alone, can clean it in 12 minutes. how long will it take them if they work together?

  6 2/3 minutes

Step-by-step explanation:

Their rates in "jobs per hour" are ...

  (60 min/h)/(15 min/job) = 4 jobs/h

and

  (60 min/h)/(12 min/job) = 5 jobs/h

So, their combined rate is ...

  (4 jobs/h) + (5 jobs/h) = 9 jobs/h

The time required (in minutes) is ...

  (60 min/h)/(9 jobs/h) = (60/9) min = 6 2/3 min

Working together, it will take them 6 2/3 minutes.

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step-by-step explanation:

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