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Can You Solve The Math Exam Question For Students In Singapore That Went Viral?

April 13, 2015

This creative math problem on a Singapore exam for students aged 15-16 is stumping adults across the world.

This is the question:

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To clear up any ambiguity, Cheryl tells Albert the month in which her birthday falls, and she tells Bernard the day's number. In other words, Albert is told either May, June, July or August. Bernard is told either 14, 15, 16, 17, 18 or 19.

The answer is written out below, but see if you can figure it out first!

This is the correct answer and how it works out:

1. Albert doesn't know immediately - this gives us nothing by itself, because there are no single month choices.
2. Albert knows that Bernard doesn't know - this eliminates May and June, because were it either of those, there would have been a chance the date was May 19th or June 18th (which are unique dates), in which case Albert wouldn't have known definitively that Bernard doesn't know.
3.Bernard knows after deducing that May and June have been eliminated - this eliminates July 14 and August 14, because were it either of those dates, he would not have been able to say he knew for sure.
4.This leaves us with July 16, August 15, and August 17. After hearing Bernard knows, Albert also says that he knows - this is only possible if the remaining choice is unambiguous in terms of month. This eliminates the duplicate August dates, leaving us with July 16 as the answer.