If we told you that there are a number of people gathered in a room next door and that there’s better than a 1-in-2 chance that two of them share the same birthday. How many people would you think are in that room?
The number is in fact quite small — just 23. In a room of 75, there’s a 99.9% chance of two people matching.
This is what’s known as the birthday problem. More specifically, it refers to the chances that any two people in a given group share a birthday. The reason it is a “problem” is that most people — puzzle lovers and math majors excepted — tend to underestimate its likelihood.
Let’s see why the paradox happens and how it works.Source: https://betterexplained.com/articles/und...