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"It's 'the _th floor', not 'floor X'. That's the trick."
"That's right. Though, in a way, the answer is right there in the question."
"What do you mean...?"
"For example, let's take floor 7. You'd call that 'the seventh floor', right? But you wouldn't call floor 3 'the threeth floor', would you?"
"Precisely. From floor 1 to floor 4, there's only one floor that ends in 'th'."
"A, ah..."
By this point, even I had figured out the answer. If only I'd paid a bit more attention to the way Krauss oji-san said the question...
"Hahahahahahaha. Too bad. Looks like I'll be keeping this medal."