Life lesson learned for OP, when it comes to tech interviewing:

(1) the riddles you already know you answer after some staged deliberation to pretend you are working it out from first principles using your genius brain (bonus points if you can work "first principles" into the conversation).

(2) The riddles you don't know the answer to are the ones you say you already read in a book or online and repeat this until you get back to a (1)

This was the real riddle all along.

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