Question

Is there any rule to find a no. is prime or not?

Answer 1

Yes, it can only be divided by itself and 1. What I mean is for example. 3 is a prime. You can divide it by 3 and 1 to get a whole number. 4 is not, you can divide it by 1,2 and 4

Answer 2

Not in a hurry. In principle, here's what you have to do. (This procedure is especially useful when you're letting a computer do it. I've examined all of
the numbers up to several million this way):

Check every integer, from 2 up to 1/2 of the number you're wondering about.

For each one, divide it into the number you're wondering about, to see whether
the smaller number is a factor of the bigger one.

If you make it all the way from 2 up to 1/2 of the big number without finding
a factor, then the big number is prime.