composite probability (with solution)

06/01/2013 10:10 go for it#1
a year ago i've done wrong composite probability equation which i couldn't fix and i've had another look today and got it solved , it's was fun solving it which to why i shared it

3 red marbles and 4 blue marbles. You put them all in a bag and without looking you pull out three marbles. What is the probability that all three marbles are red?

take your time thinking and then open the spoiler

:cool::cool::cool:
06/02/2013 17:12 CptSky#2
You have one chance on the number of permutations of the objects. But, objects are similar, you must not count few permutations several times.

Following the theorem, it's :
06/02/2013 20:04 go for it#3
Quote:
Originally Posted by CptSky View Post
You have one chance on the number of permutations of the objects. But, objects are similar, you must not count few permutations several times.

Following the theorem, it's :
you mean 3! 4! / (3+4)! ?
06/02/2013 20:11 CptSky#4
Quote:
Originally Posted by go for it View Post
you mean 3! 4! / (3+4)! ?
No, I missed the inverse when writing :p The answer is : 1 / ((3 + 4)! / (3! 4!)) = 1 / 35

(3 + 4)! / (3! 4!) will gives you the number of possible permutation where few objects are identical. So, AAABBBB is 1 possibility and not 3 (like A1A2A3BBBB could be). It's the way I would do it.