hard question(no clue how to answer)

[atreides]

Probably there's a hole in this reasoning, but I'm thinking that if you're asked to compute this problem on an interview that the solution might be something clever like this.

That, or "The most clever solutions are usually the most beautiful. They're also usually wrong."

For an interview question, maybe they didn't expect you to answer it correctly..they might have just wanted to see your thought process and the sort of assumptions you make in tackling tough problems..or maybe if they really wanted a correct answer, it could have warranted a 'grand epiphany' or an 'aha' moment during the interview.