Getting Around

Saturday, August 18, 2012

Divisibility War

The math department at my new gig decided to start the year off teaching divisibility rules. I teach the seventh grade "regular" students, and they usually need a lot of practice. Since they all seemed to know 2, 5, and 10, we zoomed through the rest of the rules and decided to play a game.

I tried to find a good game to practice divisibility, but all I could find was one involving rocks (why rocks???). So, I used my go-to backup, War.

I think every math teacher in the universe knows how to play War, so this is just a variation. All you need is a piece of paper and a pencil for each kid and a deck of cards for each group. I would like to think I invented this, but I can't imagine that someone, somewhere hasn't done it before and better.

Here's how it works (for 2 to 4ish players):

1. Divide the deck up roughly evenly, taking out the face cards and Jokers.

2. Everyone flips over two cards, putting them side-by-side.  Each card represents a digit, so a 4 and a 6 are 46.  If you get a 10, just use both digits, so a 10 and a 3 would be 103.

3. Using a chart on a piece of paper, write down your number and then make a check in the column for if it's divisible by 2, 3, 4, 5, 6, 8, 9, or 10.

4. Whoever has the most check marks wins the round and keeps everyone's cards.  Players can check each other's charts. If an error is found, the person finding the error wins. If there's a tie, the person with the smallest number wins. If there's still a tie, the winners divide the cards evenly.

5. When all the cards are played, the person holding the most collected cards wins the game. Shuffle and start again, because we all need more practice.

6. When doing it with two cards gets easy, switch the three cards/digits or more.

I never got to it because we ran out of time, but I would have let my enrichment kids change the order of the digits in order to maximize the number of factors.

The other math teachers decided not to cover the rules for 4 and 8, but I find them very handy for factoring. I told my students they were bonus rules that I taught them only because they were smarter than the other seventh graders.


  1. I found your blog through a list of middle school math blogs, and I have to say that I've never seen anyone who used the game of War in so many different ways! You've definitely given me a go-to idea for working with my kids. :P

  2. The beauty of it is that as soon as they know it's War, you don't have to give them any rules. They just need to know the parameters for winning. It cuts down on "this is what I want you to do" time.