July 16.当A说他不知道，B也不知道时，就可以排除May和June.否则B有可能被告知是18或19号因而知道答案。这时B说原来不知道，现在知道了，就可能是July 16,August 15或August 17。这时A说他也知道了，就肯定是July 16.
CNNed 发表评论于 2015-04-14 19:01:16
Here is the way of thinking:
0515 0516 0519
0617 0618
0714 0716
0814 0815 0817
Assumption: A knew the Month, and B knew the date
Solution:
1. A didn't know, but A knew B didn't know either -- so it could not be in 05 neither 06
2. Then B said B knew now - because B didn't have 14
3. Now A said A knew now - it could not be 08
So Finally, the answer is 0716
CNNed 发表评论于 2015-04-14 18:46:23
The problem is only solvable, when Cheryl told Albert the month and Bernard the date, as her birthday on July 16.
When Albert knew the date and Bernard the month, it is an unsolvable problem.
１）Ｂ不知，说明18，19不是，这样知剩下14，15，16，17。
２）Ａ知道Ｂ不知，仍然无法选择，说明不是６月。
３）Ｂ知道不是６月后，立刻就知道答案了，说明只能是１７日，因１４，１５，１６都还是双选。
４）A知道Ｂ只剩单选，就也知道了。