If a number is divisible by another number, then it is also divisible by all the factors
of that number.

80 = 10 x 8
996ab is divisible by 80. Since 10 and 8 are factors of 80,
996ab is divisible by 10 and 996ab is divisible by 8
996ab is divisible by 10. We know that A number is divisible by 10 if the last digit is 0.
Hence b = 0
Thus we have the number 996a0 which is divisible by 8
=> 6a0 is divisible by 8
=> a = 0 or 4 or 8
Hence, (a + b)
= (0 + 0) or (4 + 0) or (8 + 0)
= 0 or 4 or 8