What is a good way of guessing if a number is prime or not?

Question

I myself check if it's even, if it ends in 5, digits add up to something else than a multiple of 3 and check if it's divisible by 7 mentally

Is this a good way? It is faster than it sounds like.

Answer 1

a prime number is a number that can only be divided by one and itself, therefore, i don't think you're thinking of the right thing. trust me i know, i'm in ninth grade and i'm in geometry.

Answer 2

If the number is less than 121, you method works fine. If it is 121 or bigger, you've to take care of 11, 13, etc. There are more prime factors to watch out for.

Answer 3

yes thats the best way..try checking out the seive of erastosthanus..in wikipedia..its worth looking..

Answer 4

Actually it is not enough, for example, 143 is 13*11 which is not a prime, but if you check it like your method, it will come out as a prime. So I suggest that you try to divide the number by all prime number smaller or equals to the square root of that number. It is of course takes more long time, but you can be more sure whether it is a prime or not.