The answer to how much flour he'll have left over is actually 14 cups/

This is how you work it out:

He has no flour ("Rodney is out of flour, and goes to the store")

He goes to the store and buys a bag containing 20 cups

He needs to make 3 pizzas ("Rodney is making three pizzas")

Each pizza will use up 2 cups of flour ("The recipe says that, for one pizza, he needs 2 cups of flour")

All the other information is just thrown in to confused you.

The equation you can create to solve the problem is:

= 20 - (2 * 3)

= 20 - 6

= 14