= = = = = First of all, let's translate the title = = = = = =

Albertz and Bernade wanted to know Cheryl's birthday, so Cheryl gave them the date 10.

May 15 respectively? May 16? May 19

June 17? June 18

July 14? July 16

August 14? August 15? June 65438, 2007

Albert knows when Cheryl's birthday is, but he doesn't know when.

Bernard knows when Cheryl's birthday is, but he doesn't know when.

Albert said: I don't know when Cheryl's birthday is, but I know Bernard doesn't know either.

Bernard said: I didn't know when Cheryl's birthday was at first, but now I know.

Albert said, then I know.

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Analyze Albert's words

Albert knows when Cheryl's birthday is, but he doesn't know when.

He said, "I don't know when Cheryl's birthday is." Let's take this sentence as nonsense for the time being.

But I know Bernard doesn't know? Please note that Bernard only knows Cheryl's birthday.

It can be seen that May 19 is impossible. If it is May of 19, Bernard can only infer his birthday by the date of his birthday.

Then why does he know "neither does Bernard"? It means that the month he got is definitely not May. If it is May, Bernard may infer that the birthday (information 19) is definitely not June. If it is June, Bernard may be able to infer his birthday only by the date of his birthday (get information 18).

So, relying on Albert's words, Bernard knew that Cheryl's birthday was not in May.

Does this not include May 15? May 16? May 19? June 17? June 18

keep

July 14? July 16

August 14? August 15? June 65438, 2007

Then he said, "I didn't know when Cheryl's birthday was at first, but now I know."

Why does he know? Explain that he can launch a birthday only by relying on date information, that is, excluding

July 14? August 14, it is impossible to introduce the birthdays of these dates only by date, because if you know it is 14, it may be July or August, and you are not sure.

So the date can only be July 16? August15 August 17? one of them

Then Albert said, then I know, too

Note that at this time, both sides launched Cheryl's birthday in? July 16? August15 August 17? One of them, everyone knows the real date. Explain that this date is the only one of the three, whether it is on the month or on the day.

So it can only be? July 16

= = = = = = = = = = The above is my own answer. Look at the official answer = = = = = = = =

In these ten days, only 18 and 19 appeared once. If Cheryl's birthday is 18 or 19, Bernade who knows the date can guess the month and must know when Cheryl's birthday is. Why is albertz sure Bernade doesn't know Cheryl's birthday? As mentioned above, since May and June both have dates of 18 and 19, albertz who knows the month can judge whether Bernade is sure, so her birthday must be July or August. Bernade's words also provided information, because in the remaining five days in July and August, only 14 appeared twice. If Cheryl tells Bernade that her birthday is 14, then Bernade can't guess her birthday by albertz's words. So the only possible dates are July 16, August 15 and August 17. After Bernade's speech, albertz also knew Cheryl's birthday, reflecting that Cheryl's birthday month can't be in August, because there are two possible days in August and only one in July. So the answer is July 16.