I understand why 18 or 19 can't be the date. I understand why June can't be the month (because then only the 17th is left.) I don't understand why this rules out May 15th and 16th? Can someone explain it?
If Albert is told "May" then it's possible that Bernard is told "19", and then Bernard would know the answer.
But Bernard doesn't know the answer initially, because Albert tells us he knows that. Therefore Albert could not have been told "May"; if he was, then he wouldn't be sure that Bernard didn't know the answer.
Let's put it the other way around. Suppose Albert is told "May". Then he says to himself "Bernard might have been told 19, so he might know the answer."
But in fact he says that he knows that Bernard doesn't know the answer. So we have to conclude that Albert wasn't told "May".
Paul Clapham wrote:But Bernard doesn't know the answer initially, because Albert tells us he knows that. Therefore Albert could not have been told "May"; if he was, then he wouldn't be sure that Bernard didn't know the answer.
'm still not there. Albert knows Bernard wasn't told 19 because then Bernard would know the answer.
I know Albert can't assume Bernard wasn't told 15 or 16. In fact, he has to assume Bernard was told 15, 16 or 17. I don't see how knowing Bernard wasn't told 19 lets us assume he wasn't told 15/16.
Let's put it the other way around. Suppose Albert is told "May". Then he says to himself "Bernard might have been told 19, so he might know the answer."
Bernard might have been told 19. But he might have been told 15.
Oh! I get it now. The key is that Albert knows that Bernard doesn't know *based* on the month Albert is given.
Yes, that's right. We assume Albert was told May, and that leads to a contradiction between what he knows (based on May) and what he actually says. The contradiction means that the assumption was wrong (yeah, I'm a math person) and therefore Albert wasn't told May. (Same reasoning tells us he wasn't told June either.)
As Albert knows that Bernard dont know the birthday ,(Albert has been told that month which has date which has occured more then once in the list of 10)
Now Bernard understood it,and he got to know (it cant be may or june as both have date which occured only once in list of 10)
After this calculation he got the answer means he has bern told a date which occured once in july or aug
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