1. A Prime Number is defined as any number that can only be divided evenly by itself and 1. One digit Primes are 1, 2, 3, 5, 7 since they can only be divided evenly by 1 and themselves. .
2. A Prime Number, if it is a Prime Number, has 1, 3, 7, 9 in its’ far right column. (11, 13, 17, 19) are all Primes since they can only be divided by 1 and themselves..
3. Except for single digit prime number 3 ( see 1.), if you add the digits forming a prime number ending in 1, 3, 7, 9, a prime number’s digits will never total 3 or be a multiple of 3. For instance 69 is not a Prime because its’ digits 6 and 9 total 15 which is divisible evenly by 3. ( 69, 6 + 9 = 15, 15 / 3 = 5 ).
More Confusion:
1. Not all numbers having 1, 3, 7, 9 in their far right column are Prime Numbers. ( 21, 33, 27, 39 ) are all divisible by 3 (My lazy example). The rule is divisible by any old number except 1, since 1 never changes the number it’s dividing. The total of their digits ( 3, 6, 9 12 ) are all divisible by 3 ( see 3, last section ).
2.A number which ends in 1, 3, 7, 9 and isn’t a prime, is divisible by a number ending in 1, 3, 7, or 9 in its’ first column ( 469, 469 / 7 = 67, 7909, 7909 / 11 = 719 ).
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