Category:

# 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.

# Follow wiseGEEK: 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?