Help Please: Cool Mathematical Probability

Question

Hi there,
My assignment is to prove that in a group of 30 people, at least 50% have the same birthdays. And in a group of 50 people, 70% have the same birthdays.
I have to use Math.random()*365 for every day of the year and store the values in an array of 30 and 50. I have to check of how many times each array has the same numbers. This program has to run 100,000 times
When you compile and run the program, this is what you should get:
50% 30 people
70% 50 people
This is all I have so far, but I'm stuck and don't know how to check the arrays.
import jpb.*;
public class Project5
public static void main (String []args)
     int[] a = new int[30];
     int[] b = new int[50];
     int ran1 = 0;
     for(int i=0; i<100000; i++)
     for(int cnt=0; cnt<30; cnt++)
          ran1 = (int) Math.random()*365;
          a[cnt]= ran1;

Answer 1

Well, one the one hand it's a simple probability problem. On the other hand, my statistics is REAL weak. Fortunately, my web skills are high, thus turning up:
http://www.studyworksonline.com/cda/content/explorations/0,1035,NAV2-76_SEP949,00.html
Anyway, here's my code. I know it's homework, but it was quick and painless (save for the results which didn't match the original posters, but they match the web site above, so "Works for me!").
Enjoy!
Make sure you cite me as a reference when you turn it in!
import java.util.Arrays;
import java.util.Random;
public class BDay
    Random r;
    public BDay()
        r = new Random();
    public boolean matchingBDays(int numberOfPeople)
        int bdayList[] = new int[numberOfPeople];
        int i;
        boolean result = false;
        for(i = 0; i < numberOfPeople; i++) {
            bdayList[i] = r.nextInt(365);
        Arrays.sort(bdayList);
        for(i = 1; i < numberOfPeople; i++) {
            if (bdayList[i] == bdayList[i - 1]) {
                result = true;
                break;
        return result;
    public float percentMatching(int iterations, int numberOfPeople)
        float successes = 0;
        float result;
        int i;
        for(i = 0; i < iterations; i++) {
            if (matchingBDays(numberOfPeople)) {
                successes++;
        result = successes/(float)iterations;
        return result * 100;
    public static void main(String args[])
        BDay b = new BDay();
        System.out.println("100,000 iterations with 25 people:" +
                           b.percentMatching(100000, 25));
        System.out.println("100,000 iterations with 60 people:" +
                           b.percentMatching(100000, 60));