How Many Prime Numbers End with a 2 or 5?

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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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.


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.

Post 1

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

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