After writing about the importance of memorizing your math facts, I kept changing my mind about what to bring up next. At first, I thought I’d go over some fraction tricks, but I realized it would be better to go over least common multiples first. Then it hit me, the two things I mentioned before, along with a lot of other concepts in math are drastically easier if you know your divisibility rules!
I’ll slow down. Today I’m going to talk about how to tell if a number is divisible by 2 or 3.
I’ll anticipate some questions and answer them ahead of time.
Question: I’m already lost. What does divisible mean?
So glad you asked! A number is divisible by any number that it can divide into equal groups of, with no remainder.
Here’s an example: 8 is divisible by 2 because when I do 8 divided by 2 I get 4 with no remainder. 8 is not divisible by 3 because if I do 8 divided by 3 I get 2 with a remainder of 2.
Question: What is a divisibility rule?
A divisibility rule is a quick way to tell if a large number is going to be divisible by a single-digit number without having to divide it out.
Question: Will this ever come in handy?
YES. Very frequently. It is also a really great time saver for standardized tests.
A number is divisible by 2 if it is even. That means it ends in a 0, 2, 4, 6, or 8.
Is 674,522 divisible by 2?
Yes! It ends in a 2, which means its an even number.
Is 700,000,001 divisible by 2?
No! It’s an odd number.
A number is divisible by 3 if the sum of its digits are divisible by 3.
Is 108 divisible by 3?
We have to add the digits together first. 1 +0+8= 9. Since 9 is divisible by 3, then 108 is divisible by 3.
Is 1,987 divisible by 3?
1+9+8+7= 25. Since 25 is NOT divisible by 3, 1,987 is not either.
Stay tuned for rest of the rules, along with worksheets and handouts!