Chicken Math is when you go to the feedstore to pick up your order, and they tell you that they have 25, even though you only ordered 20. There wasn't anyone else who ordered this type of chick, so they can all go with you, if you'd like to pay for those extra 5.
And you say "Sure. Why not? I'm already getting 20....and they don't have anywhere else to go. What's 5 more?"