RP Problem of the Week: Math & Logic

RoundPier Team
September 30th, 2017

In our Eduniverse section we will publish various problems and questions in math, science and other disciplines. We collect these from around the world as well as have our own team creating various interesting problems for our members.

This week we start with the problem that some of you may have heard of. It’s called Cheryl’s Birthday. Initially introduced at the Math Olympiad in Singapore the problem became popular around the world thanks to a Facebook post by a local TV presenter.

We thought the original wording of the problem is very confusing and have amended it for our members.

Cheryl’s Birthday

Albert and Bernard just met Cheryl. The boys wanted to know when her birthday is.

Cheryl did not tell them but she wrote down a list of 10 possible dates:
May 15, 16, 19
June 17, 18
July 14, 16
August 14, 15, 17

Then Cheryl told Albert the month but not the day of her birthday. And told Bernard the day but not the month of her birthday.

The boys made the following statements after hearing this.

Albert: I don’t know when Cheryl’s birthday is, but I know for sure that Bernard doesn’t know it, either.

Bernard: I didn’t know originally, but now I do.

Albert: Well, now I know, too!

When is Cheryl’s birthday?

Register at roundpier.com to see the solution