Question

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?

Answer 1

Answer:

  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.