The birthday paradox: why 23 people are enough to share a birthday
How to calculate the birthday paradox probability, one person at a time
Each pair of people in a group shares a birthday 1 time in 365, but adding up those chances gives the wrong answer. So how do you count the real chance that two people share a birthday, with no formula?
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1
Adding cannot be right. With 28 people, the pairs would add up to 104 chances in 100: more than sure. The pairs share people: Miva alone is in 27 of them. So adding counts some chances more than once.
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2
Rono turns the question around: what is the chance that nobody matches at all? He hangs a calendar of 365 days and marks each birthday as he reads it. The first passport can land on any day.
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