What are prime numbers, you ask?
A prime number is a number that can be only be divided by itself and 1 without producing a decimal. So if you have some number, x, then only x/x and x/1 will produce whole numbers, then you have a prime. If some number, x has x/a is a whole number, where a≠1 or x.
So 5 is a prime because only 5/1 and 5/5 equal whole numbers. 5/2, 5/3, and 5/4 all equal numbers that are not whole numbers, but numbers with decimals.
Prime numbers are also always odd, with the exception of 2. The reason is that all even numbers are multiples of 2, and so can be divided by 2 to produce a whole number. For example, 86 cannot be prime because it can be divided evenly by 2. However, not any odd number will do. Only numbers ending in 3,7,and 9 can possibly be prime. The reason is that all numbers ending in a 5 are multiples of 5 and therefore not prime. But that does NOT mean that numbers ending in 3,7, or 9 are automatically prime. Quite the opposite turns out to be the case.
For example, 7328497329372643274863489327640 is an even number, so it can automatically be divided by 2 to produce a whole number. In fact, this particular number can also be divided by 5 and 10 and possibly other numbers. Nonetheless, it is NOT a prime number. So if a number is even (ends in 2,4,6,8, or 0) it is automatically NOT prime because it can be divided evenly by at least 2.
Now look at 7623. It sure looks prime. It ends in an odd number that isn’t 5 or 2, but it maybe could be divided evenly by 3. ’Oddly’ (pun intended) enough it can be divided by 3 as well as 77 and 99 (and maybe other numbers too). So don’t let looks deceive you with prime numbers. Many times numbers look prime and are not.