Play Common Denominator War

What You Do:

1. Write numbers on at least 20 index cards, making sure that most of the numbers are not prime. For example, you might choose numbers such as 8 (which has the factors 2 and 4) or 9 (which has 3 as a factor) more often than 7 or 11, which are both prime. Most of the numbers should be between 1 and 20, with a few larger ones that have a lot of factors (e.g., 20, 24, 30).
2. Divide the pile of index cards in half into two smaller piles, and give each player one of these smaller piles.
3. Both players call out “1, 2, 3…war!” and simultaneously turn over the top card in their piles and place it between them.
4. The goal: as quickly as each player can, calculate the lowest common multiple of the numbers on the two cards, and call it out.
5. The first player to call out the correct lowest common multiple wins the round and gets to add both of the cards to the bottom of her pile. Repeat Steps 3 and 4 for the next round.
6. The game ends when one player runs out of cards. The other player is the winner.