Divisibility rules: Your best friend when reducing fractions

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

  1. Everything. All the time.
  2. Digit in the ones place is a 0, 2, 4, 6, or 8.
  3. The sum of the digits is divisible by three.
  4. Last two digits are divisible by four.
  5. Digit in the ones place is a 5 or a 0.
  6. Both 2 and 3 are factors
  7. Don’t worry about it.
  8. Last three digits are divisible by 8.  On second hand, don’t worry about it.
  9. The sum of the digits is divisible by nine.
  10. 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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