Cheryl’s Birthday – Singapore Math Problem

This math problem went viral yesterday so I had my kids tackle it. It took us all working together but we got a solution.

Here’s the problem that appeared all over reddit, Facebook, and Twitter:

  

Don’t read any further unless you want to know the answer. #spoilers

Hint 1: Albert’s first statement rules out any month with a unique day (18 or 19) since he’s certain Bernard doesn’t know the exact date. If Bernard had been told 18, for example, that means he could know it was June 18, but Albert is certain Bernard doesn’t know so it couldn’t have been June that Albert was told. Same goes for May since the 19th is unique to May. 

Hint 2: With May and June ruled out, Bernard says he now knows what the full date is. So the day couldn’t have been one that is in multiple months (i.e., 14 is in July and August). So, the 14’s are ruled out. 

Final Hint (aka the solution): Since Albert finally says that he knows the date and we’ve ruled out May, June, and the 14th’s, we know it has to be a month with only one date left, namely July 16. Since August still has two choices, the 15th and 17th, Albert wouldn’t have been certain that he knew the date if he had been told August. So Albert must have been told July. 

Final answer: July 16 is Cheryl’s birthday. 

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