The Two Solutions of Cheryl’s Birthday Viral Math Problem

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You’ve probably already seen this math problem that went viral:

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But… did you know that it had 2 possible answers? Well, there’s one expected answer, and another one which kind of makes sense. To understand this, let me clean up the discussion between Albert and Bernard.

Albert: I know that Bernard does not know.
Bernard: I know now.
Albert: I know now too.

With this clean discussion, there’s only one possible answer — the expected answer. However, the way the text is written may be misleading, and many people on the Internet have read it as follows.

Albert: I see that Bernard does not know. And yet, I still don’t know.
Bernard: I know now.
Albert: I know now too.

In this new setting, there is still only one possible answer… but it’s not the same! Do you see the difference? Do you want to know the two answers?

Here are the two answers summed up by James Grime: