Because back then, the Koreans used a different calendar, which went by the moon instead of the one that goes by the sun.
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Prob(two sets of matched birthdays) = 1 - [Prob(No matched birthdays)] + [Prob(One set of matched birthdays)] = 0.63
No. You're only born 1 day, and can only have 1 birthday. Of course you can celebrate it on another day, and then call it 2 birthdays, but you can only have one real :)
It is to celebrate the day of your birth. and the reason why peoples 18th birthday is so important is because they are growing into an adult.
One! Why do people write such silly questions?
For the chance to be at least 50% that two people share the same birthday, there needs to be 22 people. For the chance to be exactly 100% that two people share the same birthday, there needs to be 366 people. If there was 365 people, there would be a very small chance that each person in the room would have different birthdays. With 366 people, there are not enough individual days for every person to have a different birthday, so there has to be at least one pair.