Why are all 6 - digit Fredholl numbers composite?

If we have a 6 - digit fredholl number, all numbers must consist of 3 occurrences of digits A, and 3 occurrences of digits B.

So a sum of their digits is 3a+3b

3a+3b=3 (a+b) and this number is divisible by 3,

Therefore any 6-digit fredholl number is divisible by 3.

If a number is divisible by any other number than 1 and itself, we call that number a composite number.