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.
