No, Cheryl is neither your friend nor mine. She is just part of a Primary 5 Mathematics question that has been going round in social media.
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| When is Cheryl's birthday? |
Now let's tackle this question step by step.
According to the question, Albert knows the month while Bernard knows the date.
Step 1: Draw a table like this:
Dates provided by Cheryl:
May 15
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May 16
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May 19
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June 17
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June 18
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July 14
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July 16
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August 14
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August 15
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August 17
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Step 2: Eliminate the impossible answers.
Albert said, "I don't know when Cheryl's birthday is, but I know that Bernard does not know too."
CLUE 1: Albert was quick to make the first statement and he is very sure Bernard does not know too.
Conclusion: This means that he was told "July" or "August".
If Cheryl has told Albert "May" or "June", Albert would not be 100% sure that Bernard does not know the answer as Bernard could have been told "18" or "19" which would have been conclusive.
Hence we can eliminate the 2 months.
July 14
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July 16
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August 14
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August 15
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August 17
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Now we are left with 5 possible dates.
CLUE 2: Bernard then said, "At first I don't know when Cheryl's birthday is, but I know now."
This statement eliminates 14 because if Bernard knew the date was 14, he cannot conclude Cheryl's birthday.
Hence the possible dates are now July 16, Aug 15, Aug 17.
July 16
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August 15
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August 17
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CLUE 3: Albert said,"Then I also know when Cheryl's birthday is."
This statement concludes the whole puzzle.
If the month Albert got was August, he still would not know Cheryl's birthday. Only Bernard who has the date, knows.
Hence the month has to be July in order for both to know the birthday.
CONCLUSION: Cheryl's birthday is on July 16!
Tags: Primary 5, Mathematics, When is Cheryl's birthday, Albert, Bernard, July 16
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