Math Tricks...Divisibility Rules Rule!

Numbers are our friends, not foes!  Math need not be feared, my friends.  Mathematics is an amazingly beautiful language through which God communicates so much information about His Creation. 

We NEED to impart to our children an understanding of this amazing language of computation.  They must be well-armed to face life in this world full of this…

Number sense is the understanding of numbers and their relationships with each other using basic operations. In our house, rote memorization of the math facts is essential and required.  I also like for my kiddos to know some tips and tricks to spot factors and to work quickly when they are applying multiplication in their problems.

First, we need to master some basic number sense vocabulary.

In addition, the numbers that are added are called addends and the answer is the sum.

addend + addend = sum

In subtraction, we subtract the subtrahend from the minuend to find the resulting difference.

minuend – subtrahend = difference

Numbers that are multiplied are called factors.  The result is the product.

factor x factor = product

We divide the dividend by the divisor to find the resulting quotient. 

dividend / divisor = quotient

RULES FOR FACTORS AND TESTS FOR DIVISIBILITY

2	All numbers multiplied by 2 are even.	All even numbers are divisible by 2.
3	The sum of the digits of any multiple of 3 is 3, 6, or 9.	All numbers whose digits add up to a multiple of 3 are divisible by 3.
4	All multiples of 4 are even.	All numbers whose last two digits are divisible by four are divisible by four.
5	Odd multiples of 5 are odd and even multiples of 5 are even.	A number is divisible by 5 if the last digit is 5 or 0.
6	The sum of the digits of any multiple of 6 is 3, 6, or 9.	A number is divisible by 6 if it is divisible by 2 AND 3.
7		This one’s a bit tricky, but it does work: If you double the last digit and subtract it from the rest of the number and the answer is 0 or is divisible by 7, then the original number is divisible by 7. (Note: you can apply this rule to that answer again if you want) Really, it’s often faster just to check it the old fashioned way.
8		If the last three digits are divisible by 8, then the entire number is.
9	All multiples of 9 have digits that add up to 9.	If the sum of the digits is divisible by 9, then the entire number is.

This chart is required copywork for my 5^th and 6^th graders until complete mastery.  This, in addition to rote memorization, has been very helpful to them in computing faster.

Check out a much more complete list of algorithms on the Divisibility Rules wikipedia page.

Now, go teach your children math.  Be mean. Be cruel.  Be consistent.  Release your inner dragonlady.  Your child deserves to know what’s wrong with this…