Answer: 6,760,000 different possible serial numbers.
Step-by-step explanation: There are 10 digits numbered 0 to 9. There are also 26 letters in the alphabet. Using this, we can do 10*10*10*10*26*26 as there are four digits needed and two letters. This computes to approximately 6,760,000 different possible serial numbers, assuming no other factors are involved.
