Which is not possible?

I think it's 7.

It is very easy. First of all, 12320000 is divisible by 11. Thus, we end up with the number 2ab56. And apply the divisibility rule for 11. Add the sum of digits in the odd places and you get - 8 + b Add the sum of digits in the even place and you get - 5 + a Thus, you get 2 equations = 3 + b - a = 11 or 0 The first one works with all numbers except 7 and the second one agrees with 7 as one value of b. Thus, only 7 is the answer. :)

i think 0

I think the values of b which are not possible for your expression are : 7, 8, 9.