He wasn't serious but he would have been interested in a quick answer. I didn't get it cause I couldn't think of a quick way to know what a reasonable upper bou

He wasn't serious but he would have been interested in a quick answer. I didn't get it cause I couldn't think of a quick way to know what a reasonable upper bound for the serial number is (I believe they're printed sequentially). To be fair the guy spent his whole career learning how to count things!