So, we seem to be struggling with remembering the divisibility rules. For the record, they are:

- Everything. All the time.
- Digit in the ones place is a 0, 2, 4, 6, or 8.
- The sum of the digits is divisible by three.
- Last two digits are divisible by four.
- Digit in the ones place is a 5 or a 0.
- Both 2 and 3 are factors
- Don’t worry about it.
- Last three digits are divisible by 8. On second hand, don’t worry about it.
- The sum of the digits is divisible by nine.
- Digit in the ones place is a 0.

Here’s the reason why you should memorize these rules. 1) They make it a lot easier to do a lot of math. If you can look at a number and tell what it’s divisible by, that’s the first step to solving a lot of problems. 2) Once you learn these, your brain will be faster than using a calculator. Again, we’re trying to work smarter, not harder. 3) Math is awesome. These rules are really cool and interesting. I mean seriously, we could talk about the unique application of these rules to base-10 systems for a whole class if not more. Then, even more classes on deriving new divisibility rules for other numerical systems – binary, hexadecimal….

Ok. Let’s get it back on the rails.

Here are Helton’s free-form tips for learning these rules. Try one. Try them all. Get a TicTacToe. Enjoy!

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