[Darnright]

>Me too. It sure "looked prime"!<

It did, but the only thing that saved me is a rule I learned in elementary school - add the 2 integers (5+7), and if they equal a number divisible by 3, then the higher number is also divisible by 3.

Ok I just bored the math whizzes on this thread to tears.

[xm177e2]

add the 2 integers (5+7), and if they equal a number divisible by 3, then the higher number is also divisible by 3.

(A + B == 3X) <--> (10A + B == 3X)

This is clearly true because 10A is just 3A + 3A + 3A + A, so it is an identity that 10A + B == 3X is the same as A + B == 3X + 9A which is the same as A + B == 3(X + 3A) which is A + B == 3X

This trick works because 10 is 3*3 + 1. It also works for nines because 10 is 9*1 + 1.

(A + B == 9X) is (A + B == 9(X + A)) is (9A + A + B == 9X) which is (10A + B == 9X)