## Introduction

The Birthday Paradox is a captivating probability puzzle that explores the likelihood of two or more people sharing the same birthday in a group. Initially introduced by mathematicians like Richard von Mises and Harold Davenport, the paradox has applications in cryptography, known as the birthday attack. Let’s delve into the mathematical intricacies and discover why it’s so counterintuitive.

## Scenario Analysis

Imagine attending a party with 23 friends. What’s the chance that at least two of them share the same birthday? Surprisingly, it’s around 50%. As the group size increases to 75, the probability skyrockets to an astonishing 99.95%. This might seem peculiar given there are 365 possible dates and only 75 people.

## Mathematical Framework

To grasp the logic, we start by calculating the probability of no one sharing a birthday, denoted as P(B). The complementary event, P(B’), represents the chance of at least two people sharing a birthday. The math involves determining the chance of two people having different birthdays and considering all possible pairs in the group.

## Probability Calculation

Let’s break it down for a group of 23. The chance of two people having different birthdays (P(A)) is calculated based on the remaining days in the year. The number of possible pairs (pairs) in the group is determined using a combination formula. The probability of no one sharing a birthday (P(B)) is then raised to the power of the number of pairs.

Finally, P(B’) is found by subtracting P(B) from 1, leading to the surprising result of approximately 50.05%.

## Alternative Calculation

Another approach involves considering each person entering the room one by one. The probability of not sharing a birthday with those already present diminishes progressively. The overall probability (P(B)) is obtained by multiplying these individual probabilities.

## Using the Calculator

For simplicity, a Birthday Paradox Calculator can handle the calculations effortlessly. By inputting the group size, you receive the probability of at least two individuals sharing a birthday.