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A Prime number is one that has only two factors: one and itself.

You can tell if a number is prime quickly by using the Laws of Divisibility:

  • Numbers that end in 0, 2, 4, 6, and 8 are divisible by 2.
  • If the sum of a number's digit is divisible by 3, the number itself is divisible by 3.
  • If the last two digits of a number are divisible by 4, the number itself is divisible by 4.
  • If a number ends in 0 or 5, it is divisible by 5.
  • If a number is divisible by both 2 and 3, it is divisible by 6.
  • If you double the last digit and subtract it from the rest of the number and the answer is 0 or divisible by 7, it is divisible by 7. For example, 4364: 4 doubled is 8. 436 - 8 = 424. You still cannot readily tell if the number is divisible by 7, so you can do it again: 4 doubled is 8. 42 - 8 = 34. 34 is not divisible by 7 so 436 is not divisible by 7
  • If the last three digits of a number are divisible by 8, the number itself is divisible by 8.
  • If the sum of the digits of a number is divisible by 9, the number itself is divisible by 9.
  • If a number ends with 0, it is divisible by 10.
  • If you add every other digit and then subtract the rest of the digits and the answer is 0 or is divisible by 11, the number itself is divisible by 11. For example: 1364: 1+ 6 = 7 and 7 - 3 - 4 = 0. The number is divisible by 11.
  • If the number is divisible by 3 and 4, it is divisible by 12.

There are more divisibility rules that can be used for large numbers.

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โˆ™ 2014-11-06 04:04:57
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Q: How do you tell if a number is prime or not?
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One can tell whether a number is prime if it got only two divisors which is 1 and the number itself.


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Is 2 an prime or composite number?

Yes. it is the first prime number. Whenever you wanted to know a number is prime or composite, use this site. It can tell you that: prime-calculator dot com. It has lots of other abilities. Enjoy it.


How do you test a number to see if it is a prime number?

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What does prime factorization tell you about numbers?

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Is three prime or composite number?

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