Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

With repetition allowed,

You multiply the r number of times for total n objects

n * n * n ... 

When the repetion is not allowed then you take away an object each time

n * (n - 1) * (n - 2) ....

So where does the proof come from that you have to multiply that times with number of objects?

share|improve this question
1  
Try to elaborate what the question is; currently I can't understand it –  Gadi A Jun 6 '11 at 6:36
    
Difference between the binomial distribution (replacement) and the hypergeometric distribution (without replcaement) perhaps? –  Juan S Jun 6 '11 at 6:49
    
My question is, how do you know that multiplying will get you the number of ways? is there any proof? –  user753214 Jun 6 '11 at 7:03
    
the number of ways - of what? Please don't leave a comment, instead, please edit your question so that someone other than yourself can understand what exactly you are asking. –  Gerry Myerson Jun 6 '11 at 7:22

2 Answers 2

up vote 0 down vote accepted

http://en.wikipedia.org/wiki/Rule_of_product Are you meaning this? You can resort to Cartesian product and cardinal numbers in set theory.

share|improve this answer
    
This is what I was talking about. –  user753214 Jun 6 '11 at 10:18
1  
@user753214, if you think this is a good answer, you can vote it up, you can even accept it. If it's not good enough for you to do that, you can explain what more you need. –  Gerry Myerson Jun 6 '11 at 12:45

I am assuming you are trying to pick up $r$ numbered balls from a total of $n$ balls, and you are looking for the number of ways to do this such that the ordering of the balls is relevant.

Suppose repetition is allowed, this means that you pick up one ball from the urn(?) and then put it back and then repeat the process. There are $n$ ways of doing it in the first step and for each of those $n$ ways, there are another $n$ ways of doing it in the next step and so on so forth for $r$ steps.

In the second case, when repetition is not allowed, the urn progressively loses balls, so there are $n$ ways in the first step, $n-1$ in the second, etc.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.