First of all, let's define a unit rate. A unit rate is a ratio of the amount of one measurement to one unit of another measurement term. Unit rates can be expressed as fractions.
Now, here is something easier for you to remember: <u>the second unit always have to be something with the quantity of 1</u>.
In this case, "3 cookies/bag" shows how there are three cookies per, or in each bag--which means 1 bag. The second unit, which is bag, has the quantity of one.
