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## The Math Tutor - Austin

Some Easy Divisibility Rules

So how do you know whether a number is divisible by another? For example, is 24345 evenly divisible by 45?

First - the bad news - you just don't. Not without dividing and looking at the remainder, anyway.

However, here's the good news - for some numbers, you can!

Here are some quick ways to check for some easy factors!

[2] If your number is even, it's divisible by 2

Example - 23478 is even, and therefore divisible by 2. (easy!)

[3] If the sum of the digits is divisible by 3, then the whole number is!

Example - 23478 is divisible by 3, because 2+3+4+7+8 = 24, and 24 is divisible by 3. (that is, 24 = 8*3)

(Also note - you can keep going! If you didn't know 24 was divisible by 3, you could add 2+4 = 6 - which is even more clearly divisible by 3.

(Also note - you can keep going! If you didn't know 24 was divisible by 3, you could add 2+4 = 6 - which is even more clearly divisible by 3.

[5] If the number ends in 5 or 0, it's divisible by 5

Example - 24345 is divisible by 5, because it ends in 5.

[6] If the number's divisible by 2 and 3, it's also divisible by 6

Example - 23478 is divisible by both 2 and 3, so it's also divisible by 6

[9] If the sum of the digits is divisible by 9, then the whole number is!

(this works just like the rule for 3...)

[10] If the number ends in 0, it's divisible by 10

Example - 23470 is divisible by 10. (Even more - 23470 = 2347*10)

Okay - these almost fall into the category of "Stupid Math Tricks" - but, they're simple, easy to remember, and they can be really helpful when you're trying to find the factors of a number!