How to Tell if a Number is Divisible by 2 or 3!

Knowing These Rules Makes Math So Much Easier!

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.

Let’s practice:

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!


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