In 30 boxes are 15 balls. Chance all balls in 10 or less boxes?

Question

Question1: I found 30 boxes. In 10 boxes i found 15 balls. In 20 boxes i found 0 balls. Afer i collected all 15 balls i put them randomly inside the boxes.

How much is the chance that all balls are in only 10 boxes or less?

Question2: I found 30 boxes. In 10 boxes i found 15 balls. In 20 boxes i found 0 balls. In two of the boxes i could find 3 balls. (So in one box has to be 2 balls and in the other seven boxes have to be 1 ball.) Afer i collected all 15 balls i put them randomly inside the boxes.

How much is the chance that i find in only 2 boxes 6 balls or more?

I wrote a c# programm and tried it 1 million times. My solution was: With a chance of 12,4694% all balls are in 10 boxes or less.

Answer 1

Solution of Question 1:

This is an occupancy problem with $n=30$ boxes and $k=15$ balls.

Let's first consider the expected number of empty boxes. That is much easier to obtain. The exact answer is $30(1-1/30)^{15}=18.04.$ This is approximately $30/\sqrt e.$ See the answer by Mr.Spot to this question:

Making 400k random choices from 400k samples seems to always end up with 63% distinct choices, why?

The probability of exactly $j$ empty boxes is, for $n-k\le j\le n-1$:

$$P(j)={n \choose j}\sum_{m=0}^{n-j}(-1)^m {n-j \choose m}\left(1-\frac{j+m}n\right)^k $$

See this:

http://probabilityandstats.wordpress.com/2010/04/04/a-formula-for-the-occupancy-problem/

For $n=30$ and $k=15:$

$P(20)$ to $P(24)$ is $0.096,0.024,0.0036,0.00029,0.000013,....$

and the probability of at least $20$ empty cells = 0.124371

Solution of Question 2:

You throw 15 balls into 30 boxes. What is the probability of the following result:

2 triple, 1 double and 7 single occupancy boxes and the rest empty.

$${30 \choose 2} {28 \choose 1}{27 \choose 7}\frac{15!}{3!3!2!1!^7}\frac{1}{30^{15}} $$ $$=\frac{(30)(29)...(21)}{30^{15}2!1!7!}\frac{15!}{3!3!2!1!^7} $$ First we select the 2 different boxes for the triples; of the 28 remaining boxes we select 1 for the double; of the 27 remaining boxes we select 7 for the singles. Then the multinomial coefficient gives the number of ways to assign the 15 balls to those particular boxes and there are $30^{15}$ equally likely ways to throw the 15 balls into the 30 boxes.

Answer 2

Random trials/Monte Carlo simulations are notoriously slow to converge, with an expected error inversely proportional to the square root of the number of trials.

In this case it is not hard (given a programming language that provides big integers) to do an exact count of cases. Effectively the outcomes are partitions of the 15 balls into some number of boxes (we have thirty boxes to work with, so at least half will be empty).

I wrote a Prolog program to do this (Amzi! Prolog has arbitrary precision integers built in), and got the following results:

$$ Pr(\text{10 or fewer boxes occupied}) = \frac{59486359170743424000}{30^{14}} \approx 0.124371 $$

$$ Pr(\text{2 boxes hold 6 or more balls}) = \frac{30415369655816064000}{30^{14}} \approx 0.063591 $$

The reason I'm dividing by $30^{14}$ in these probabilities is because I normalized the counting to begin with one case where a ball is in one box. If we counted that as thirty cases, we'd need to divide by $30^{15}$. So this keeps the totals slightly smaller. Each ball we add increases the total number of cases by a factor of $30$.

I wrote a recursive rule to build cases for $n+1$ balls from cases for $n$ balls. The first few cases have the following counts:

 /* case(Balls,Partition,LengthOfPartition,Count) */ 
 case(1,[1],1,1).   /* Count is nominally 1 to begin */
 case(2,[2],1,1).
 case(2,[1,1],2,29).
 case(3,[3],1,1).
 case(3,[1,2],2,87).
 case(3,[1,1,1],3,812).   /* check: for Sum = 3, sum of Count is 900 */

The number of cases generated is modest enough for a desktop, daunting to manage by hand. For $n=15$ there are $176$ partitions. It simplified the Prolog code to maintain the partitions as lists in ascending order.

Answer 3

I will assume the balls and boxes are indistinguishable.

The first problem is: If I distribute $15$ balls among $30$ boxes, what is the probability that at most $10$ boxes contain a ball?

First, the fact that there are $30$ boxes does not matter, since they are indistinguishable. So we only need to consider the problem as if there were $15$ boxes.

Second, we'll solve the problem by finding the probability that more than $10$ boxes contain a ball, and subtract that number from $1$.

There is exactly $1$ to occupy $15$ boxes with $15$ balls: $\{1,1,1\cdots\}$

There is also exactly $1$ way to occupy $14$ boxes with $15$ balls, since, again, the boxes are indistinguishable: $\{2,1,1\cdots\}$

There are $2$ ways to occupy $13$ boxes: $\{3,1,1\cdots\}$ or $\{2,2,1,1\cdots\}$

There are $3$ ways to occupy $12$ boxes, and $5$ ways to occupy $11$ (you can and should verify these numbers).

Finally, how many ways could we distribute $15$ balls over $15$ boxes? This number is exactly equal to $p(15)=176$, where $p(n)$ is the partition function.

So the answer to problem 1 is $1-(1+1+2+3+5)/176=164/176\approx93.2\%$

EDIT: I believe this answer is drastically different from the one obtained by your program for the following reason: you might have meant, in your problem statement, "what is the probability all $15$ balls are contained in $10$ particular boxes?" This significantly changes the question; in particular, the total number of boxes being $30$ is now relevant. Let me know if this is the misunderstanding.