You know the following information that the ratio of peaches to grapefruits is:

This ratio can be also written as:

You know that you must find the number of grapefruits if Aaron has 9 peaches. Then, you can fill in the following values on the double number line:
- Knowing the ratio, you need to write the corresponding value for grapefruits (190) on the first line and the corresponding value for the peaches (90) on the second line.
- Since you know that can have 9 peaches, write this value on the line that represents the number of peaches (on the square).
- Notice that:

Then, you can determine the shown in the picture:
The answer is:
Aaron has 19 grapefruits if he has 9 peaches.