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How Many Prime Numbers End with a 2 or 5?

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Prime numbers can only be divided by 1 and itself. To check whether a number is a prime number or not, it must first be divided by 2. If the number can be divided by 2 and remains a whole number, it is not a prime number. Once divided by 2, if the number is not a whole number, the number should then be divided by 3, 5, 7 and 11. If the number comes out a whole number when divided by the remaining numbers listed, then it is not a prime number. If the number is not whole number still, then it is a prime number. For example, 2, 3, 5, 7, 11, 13, 17, 19, 23 and 29 are prime numbers by the rules listed above. A quick fact; there are only two prime numbers that end with a 2 or a 5, namely 2 and 5.

**Some interesting facts about math and numbers**:

- The search engine 'Google' actually took it's name from the misspelling of a very large number, googol.
- Math equations were once written out in words because most math symbols weren't invented till the 16th century.
- Not frequently heard of, an icosagon is a shape with 20 sides.

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prime13
Post 2 |
PROBLEM with anon993011's primality test: 169 = 13 * 13 . The number 13 is a prime number. 169 is not a prime number but may be considered as a prime number according to your algorithm above. Please run it from the number 169.It will fail. Therefore your test of primality is not a good one. @tonygomis: NB Prime numbers are used inter alias in the internet transactions or communications security arsenal. So far there is no known fast and efficient primality test for huge numbers. See RSA-based cryptography and the Riemann Hypothesis-related security challenges due to its link to the distribution of prime numbers. |

anon993011
Post 1 |
Why are they called prime numbers? Why are they necessary? Why can't we just say 2 divided by plus 2 equals? |