# Simple probability question:

1. Jan 11, 2012

### kirkulator

I just started a probability course and i'm not sure if i should use a combination or permutation for this question:

How many ways are there that no two students will have the same birth date in a class size of 60?

Should i do 365C60 or 365P60?

Thanks so much!
-Amanda

2. Jan 11, 2012

### lanedance

I would actually start from basics

pick the first student, can choose from 365 unique bdays,
the 2nd can choose from unique 364..
..
the 60 can choose from unique 365-59

so you get a total of
365*364*...*(365-59)

which does this look most like?

3. Jan 11, 2012

### lanedance

Then you should check what the question is exactly asking, can you assume the students are distinuigishable, which I think you can for this case

4. Jan 11, 2012

### kirkulator

Yeah I should have thought about the distinguishable factor. combinations dont care about any particular order so they can have repeats of cases....as far as what ive learned about them...whereas permutations only count specific outcomes once [making the outcomes distinguishable] and yes, i see now that ishould use a permutation

365!/(365-60)!
which is a permutation but i dont think i was clearly thinking about the difference between permutation and combination so i was afraid i was missing a factor in the denominator. this sounds right! thanks so much for the clarification! : )

by the way; next time ill write down what i have worked out so far in the original post.

-Amanda

Last edited: Jan 11, 2012
5. Jan 11, 2012

### bbbeard

Don't forget that a few million people were born on Feb 29th....

BBB