You buy 20 cupcakes for $15 and are going to sell them for two dollars each. You can write an expression that represents how much money you will make each day that you sell cupcakes which of these quantities should be represented with a variable.

The quantity that should be represented with a variable is the number of cupcakes sold each day. Let's represent this with the variable "x".

The expression that represents how much money you will make each day is:
2x

Here, "2" represents the selling price of each cupcake, and "x" represents the number of cupcakes sold each day. By multiplying these two values, we get the total amount of money made each day.