There are 20 people in a room what is the probability that they all have different birthdays
The question sounds EASY enough, but as you'll see, the answer is EVIL.
This is one of the fiddliest sums we've ever had to think about, so let's approach it slowly. Good luck! Show
What are the chances of 2 strangers in a room having a different birthday?To keep it simple, we'll start by saying there are 365 days in every year.
What are the chances of 3 strangers in a room having a different birthday?
What are the chances of 23 strangers in a room having a different birthday?There's a reason for jumping ahead to 23!
All this stuff is quite well known, and there's a whole chapter on it in Do You Feel Lucky?. In fact if you have 30 strangers in a room there's about a 70% chance that at least two will share a birthday. But so far all the calculations have assumed there are exactly 365 days in every year. Unfortunately for us, when the book Easy Questions Evil Answers was being put together, we were asked: What are the chances that 366 strangers in a room will all have a different birthday?
So did you understand all that? We have to thank our resident expert Michael Jones for working this out for us. And just in case you can't quite comprehend how unlikely 366 strangers having different birthdays is, Michael explains it like this: Suppose every atom in the universe had a birthday and you divided them all into groups of 366, you'd need as many separate universes as there are atoms in this one before it became at all likely that somewhere in one of the universes there was a group of 366 atoms with different birthdays! And by the way... Michael was a bit worried that his answer isn't ABSOLUTELY accurate because it doesn't allow for the fact that we didn't have a Feb 29th in 1900 and there won't be one in 2100 or 2200 or 2300. If we were going to worry about that, then we'd need to use 366 fractions that had the number 146,097 on the bottom and that would be too evil even for us! We're not mad, you know.
Do You Feel Lucky? Easy Questions Evil Answers Murderous Maths Home Page What is the probability that 2 people out of 20 have the same birthday?1−0.493 = 0.507 or 50.7%. In a randomly selected group of 23 people, it is slightly more likely than not that two or more of them share a birthday.
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Probability of Shared Birthdays. or, How to Win Money in Bar Bets.. What is the probability that both will have different birthdays?Hence, the probability that two friends have different birthdays = 1 – 1365=364365.
What is the probability that out of 30 people two will have the same birthday?The probability of sharing a birthday = 1 − 0.294... = 0.706... Or a 70.6% chance, which is likely! So the probability for 30 people is about 70%.
What is the probability that 2 people in a room have the same birthday?The birthday paradox, also known as the birthday problem, states that in a random group of 23 people, there is about a 50 percent chance that two people have the same birthday.
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