Question

Mary can clean the house in 6 hours. Her younger sister Ruth can do the same job in 9 hours. In how many hours can they do the job if they work together?

Answer 1

Take the question in terms of fractions, not algebra, for a simpler approach.

Explanation:

In ONE hour, Mary can thus clean `1/6=3/18` of the house.
In ONE hour, Ruth can thus clean `1/9=2/18` of the house.
Therefore, together they can clean `5/18` of the house in an hour.

To clean the entire house, they will take `18/18-:5/18=3.6` hours. I hope this simple method helps!