I was improving some things in the blog before trying again to write the posts about Laos and Cambodia I still own you when in the social networks appeared a math problem asked to fifth grade kids in Singapore (11 years-old). Since so many people is facing problems for understanding it, even with the explained solution, I write you here my version for dummies.
The bloody problem:
The trick is to read carefully what both guys say and to PUT ONESELF IN THEIR PLACE TO UNDERSTAND IT.
Albert says he knows that Bernard doesn’t know when Cheryl’s Birthday is. Notice that there are two dates whose day is not repeated: May 19 and June 18. If Bernard (who only knows the day, not the month) know that the day is 18 or 19, then he also would know when Cheryl’s birthday is. But Albert says Bernard doesn’t know it. Albert (who only knows the month) can do such a statement because he knows that the month is July or August (we and Bernard still don’t know which one, but he already knows it). In the moment Albert open his gob and says he knows that Bernard doesn’t know the date, the latter can discard May and June.
Then is Bernard the one who continues giving clues and says that he knows now the date. Notice that still there’s one day repeated in July and August, the 14. Therefore, if Bernard knows the date, the day cannot be the 14, otherwise he would doubt between July 14 and August 14. Thus only three dates are left for both, us and Albert.
Finally is Albert the one who tells us the solution. He says that now he also knows the date. Remember that Albert only knows the month. If the month Albert knows is August, not knowing the day he would doubt between August 15 and August 17. But he says he knows the date and the only month not repeated among the three dates left is July, so the month Albert knows is July.
So Cheryl’s birthday is July 16.