• Difficulty Level : Medium
• Last Updated : 13 Apr, 2021

How many people must be there in a room to make the probability 100% that at-least two people in the room have same birthday?
Answer: 367 (since there are 366 possible birthdays, including February 29).
The above question was simple. Try the below question yourself.
How many people must be there in a room to make the probability 50% that at-least two people in the room have same birthday?
The number is surprisingly very low. In fact, we need only 70 people to make the probability 99.9 %.
Let us discuss the generalized formula.
What is the probability that two persons among n have same birthday?
Let the probability that two people in a room with n have same birthday be P(same). P(Same) can be easily evaluated in terms of P(different) where P(different) is the probability that all of them have different birthday.
P(same) = 1 – P(different)
P(different) can be written as 1 x (364/365) x (363/365) x (362/365) x …. x (1 – (n-1)/365)
How did we get the above expression?
Persons from first to last can get birthdays in following order for all birthdays to be distinct:
The first person can have any birthday among 365
The second person should have a birthday which is not same as first person
The third person should have a birthday which is not same as first two persons.
…………….
……………
The n’th person should have a birthday which is not same as any of the earlier considered (n-1) persons.
Approximation of above expression
The above expression can be approximated using Taylor’s Series. provides a first-order approximation for ex for x << 1: To apply this approximation to the first expression derived for p(different), set x = -a / 365. Thus, The above expression derived for p(different) can be written as
1 x (1 – 1/365) x (1 – 2/365) x (1 – 3/365) x …. x (1 – (n-1)/365)
By putting the value of 1 – a/365 as e-a/365, we get following.   Therefore,
p(same) = 1- p(different) An even coarser approximation is given by
p(same) By taking Log on both sides, we get the reverse formula. Using the above approximate formula, we can approximate number of people for a given probability. For example the following C++ function find() returns the smallest n for which the probability is greater than the given p.
Implementation of approximate formula.
The following is program to approximate number of people for a given probability.

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## C++

 // C++ program to approximate number of people in Birthday Paradox// problem#include #include using namespace std; // Returns approximate number of people for a given probabilityint find(double p){    return ceil(sqrt(2*365*log(1/(1-p))));} int main(){   cout << find(0.70);}

## Java

 // Java program to approximate number// of people in Birthday Paradox problemclass GFG {         // Returns approximate number of people    // for a given probability    static double find(double p) {                 return Math.ceil(Math.sqrt(2 *            365 * Math.log(1 / (1 - p))));    }         // Driver code    public static void main(String[] args)    {                 System.out.println(find(0.70));    }} // This code is contributed by Anant Agarwal.

## Python3

 # Python3 code to approximate number# of people in Birthday Paradox problemimport math # Returns approximate number of# people for a given probabilitydef find( p ):    return math.ceil(math.sqrt(2 * 365 *                     math.log(1/(1-p)))); # Driver Codeprint(find(0.70)) # This code is contributed by "Sharad_Bhardwaj".

## C#

 // C# program to approximate number// of people in Birthday Paradox problem.using System; class GFG {          // Returns approximate number of people    // for a given probability    static double find(double p) {                  return Math.Ceiling(Math.Sqrt(2 *            365 * Math.Log(1 / (1 - p))));    }          // Driver code    public static void Main()    {            Console.Write(find(0.70));    }}  // This code is contributed by nitin mittal.

## PHP

 

## Javascript

 

Output :

30

Source:
http://en.wikipedia.org/wiki/Birthday_problem
Applications:
1) Birthday Paradox is generally discussed with hashing to show importance of collision handling even for a small set of keys.
2) Birthday Attack
Below is an alternate implementation in C language :

## C

 #includeint main(){     // Assuming non-leap year    float num = 365;     float denom = 365;    float pr;    int n = 0;    printf("Probability to find : ");    scanf("%f", &pr);     float p = 1;    while (p > pr){        p *= (num/denom);        num--;        n++;    }     printf("\nTotal no. of people out of which there "          " is %0.1f probability that two of them "          "have same birthdays is %d ", p, n);     return 0;}