The problem even explains how to determine whether or not a given number is divisible by 8. Only the final three digits matter, so determining if 992,466,1A6 is divisible by 8 is the same question as asking if 1A6 is divisible by 8.
Let's check all the digits.
Is 116 divisible by 8? No.
Is 126 divisible by 8? No.
Is 136 divisible by 8? Yes!
Of course, I could keep going and check the rest of the digits, but I have a small shortcut that saves a little bit of time. Since we are changing the tens digit, the next number that will be divisible by 8 again will be the current number + 40. Why? Well, 40 is divisible by 8, as well, so adding 40 to a number divisible by 8 will still yield a number divisible by 8.
136 is divisible by 8
176 is divisible by 8
216 is divisible by 8, but that is not allowed since A is a digit, and digits range from 0-9.
Therefore, when A = 3 and A = 7, 992,466,1A6 is divisible by 8.
The sum of all possible A's is 3+7 = 10.