Deal 5 cards to each person face down so the other player cannot see them. Decide if you will be able to use your cards multiple times or if you can only use each number once. Get pencil and paper if you need scratch paper.
Collect the most challenge cards at the end of the game.
From the remaining cards, each player chooses 1 card for the other person to make an equation for with the 5 cards that were dealt to them. They "throw down" their challenge card at the same time, and all players flip over their 5 cards and try to make their equations at the same time. If you are able to make an equation for the other player's challenge card, you get to keep it. If you cannot, that card gets discarded. Start the next round with 5 new cards for each player. The first person who collects 5 challenge cards or has the most cards at the end of the game wins.