this is a simple problem but i cant figure out how to set it up if u have 6 items of x and 9 items of y out of 3 tries what is the probability of getting atleast 1 of x now i know the bottom is 15 choose 3 but i dont know how to set up the top equation

quote:

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Original post by omegasyphon
out of 3 tries what is the probability of getting atleast 1 of x

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It''s 1 - probability of not getting an x at all
Not getting x at all means that all 3 were y
So it 1 - P(all 3 are y)
Now it depends on what probability function you''re using to determine the value for that. But that gives you the idea.

well i came up with 47%

9 6
2 1
----
15
3

does this equation seem correct?

[edited by - omegasyphon on March 14, 2002 6:36:26 PM]

probability on the first pick (of getting an x) is:

6/15 (right?)

and if you did NOT get an x the first try ... prob ont he second try is:

6/14 (right?)

and if you still hadn''t gotten one, then prob on last try is:

6/13

... SO

you add the probabilities for the tries (after adjusting for the attempts likelihood of relevance ... like this:

xOnFirst = 6/15

noXOnFirst = 1 - 6/15

xOnSecond = 6/14

noXOnSecond = 1 - 6/14

adjustedXOnSecond = noXOnFirst * xOnSecond

xOnThird = 6/13

noXOnFirstOrSecond = noXOnFirst - adjustedXOnSecond)

adjustedXOnThird = noXOnFirstOrSecond * xOnThird

so:

totalProb = xOnFirst + adjustedXOnSecond + adjustedXOnThird


I think ... i know that''s a crazy long way to do it ... just wanted to show you a way to THINK about the problem ... there are simplifications that make this system easy to implement in a for loop.

good one Xai..so where is the answer?

I understand what you mean omegasyphon, simple, but strange concepts:
If you throw a coin more times then it will give you an increasingly amount of chance (in total) of getting at least one head......so you get less of getting a tails?
No...its the same

chance of not getting head: 0.5
chance of not getting head again: 0.5
0.5x0.5=0.25
1-0.25=0.75
0.75 chance of getting at least one head out of two throws.
Is this right, did I just confuse myself? look at all those smileys.

1-((1/15x9)^3)=0.784
please, dont quote me on that!

[edited by - stevenmarky on March 14, 2002 7:10:03 PM]

well the formula i posted means this


9! 6!
----- * ------
2!*7! 1!*5!
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15!
-------
3!*12!