# Probability

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A group of 30 people gather in a room. What is the probability that at least 2 of these people have the same birthday? The year of birth is not considered; having the same birthday means two peple were born on the same day of the year.

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A group of 30 people gather in a room. What is the probability that at least 2 of these people have the same birthday? The year of birth is not considered; having the same birthday means two peple were born on the same day of the year.

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Solution. We consider one year has 365 days. Then there are 365*365*...*365=365^30 possible cases for these 30 people's birthdays since each of ...

###### Education

- BSc , Wuhan Univ. China
- MA, Shandong Univ.

###### Recent Feedback

- "Your solution, looks excellent. I recognize things from previous chapters. I have seen the standard deviation formula you used to get 5.154. I do understand the Central Limit Theorem needs the sample size (n) to be greater than 30, we have 100. I do understand the sample mean(s) of the population will follow a normal distribution, and that CLT states the sample mean of population is the population (mean), we have 143.74. But when and WHY do we use the standard deviation formula where you got 5.154. WHEN & Why use standard deviation of the sample mean. I don't understand, why don't we simply use the "100" I understand that standard deviation is the square root of variance. I do understand that the variance is the square of the differences of each sample data value minus the mean. But somehow, why not use 100, why use standard deviation of sample mean? Please help explain."
- "excellent work"
- "Thank you so much for all of your help!!! I will be posting another assignment. Please let me know (once posted), if the credits I'm offering is enough or you ! Thanks again!"
- "Thank you"
- "Thank you very much for your valuable time and assistance!"

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