Teaching Divisibility Rules

So we are finishing up our unit on multiplication and division (remainders as fractions & decimals, division by multi-digit numbers, etc.), and will complete the unit with divisibilty rules.  I don't know about you, but I cannot remember a single teacher ever teaching me these rules.  To be honest, I didn't even know they existed (as a set of rules) until I began teaching math.  If you don't have to teach divisibility rules, or you just want a quick refresher, here they are:

2:  If the last digit is even
3: If the sum of the digits is divisible by 3
4: If the last two digits form a number divisible by 4
5. If the last digit is 5 or 0
6: If the number is divisible by both 2 and 3
7: Take the last digit, double it, and subtract it from the rest of the number; if the answer is divisible by 7
8: If the last three digits form a number divisible by 8
9: If the sum of the digits is divisible by 9


I don't know about you, but some of my students have difficulty even dividing 3-digits by 1, so to ask them to do the mental math for 7 or 8 is a huge challenge. Even if they get an answer, the possibility of making an error along the way is so great, that it is almost easier for them to just do the long division.  Regardless, the curriculum demands it, so I wanted to pass something along.  While searching through Smart Exchange (the website for Smart users, but can be for anyone--no sign-up fees), I found an AMAZING presentation on these rules.  Whoever put this together gets my respect medal of the day.  I hope this helps you, if you are teaching these rules!

http://exchange.smarttech.com/details.html?id=db76013b-040a-4fa5-8703-8af6942c4793

Until the next bell...
K.