Rina recipe uses 2 cups of sugar to make 2 1/2 dozen cookies. Jonah's recipe uses 2 1/4 cups of sugar to make 3 dozens cookies. Which recipe use more sugar for a dozen cookies? Why?

Let's calculate the "cups of sugar per dozen cookies" ratio:

2 cups
---------- = 0.8  (Rina)        
   2.5                                 

2.25 cups
------------- = 0.75 (Jonah)
      3

Rina uses more sugar for a dozen cookies.

The key here is to "compare unit rates."