qr help

Doctor PJ said:

can anyone tell me how this is done, its from topscore qr.
what is the probability of getting 6 tails out of 9 tosses of a coin?

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I would do probability of getting 6 tails of 6 tosses, (1/2)^6 and multiply that by the next 3 flips getting heads * (1/2)^3
(1/2)^6 *(1/2)^3

i think?

You would use the sucess failure formula.

9c6 * (1/2)^6 * (1/2)^3

NcR * Probability of success raised to the power of the number of successes*Probability of failure raised to the power of the number of failures.

1/6 i think without doing the calculation just multiplying in my head. This is he proper way to do this.

chances of getting 6 tails appearing in 9 flips

84/512 -- 16%

Yes its 9*8*7/3*2 (1/2)9 = 84/512 = 1/6=16%

HoangDDS said:

chances of getting 6 tails appearing in 9 flips

84/512 -- 16%

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just wanted to add. I might be doing this all wrong and unnecessarily long, but I am not sure how to do any other way. Can someone please explain how that NcR success failure works? Thanks.

this is how I did it

9! / [(9-6)!(6!) = 84 , which means there is 84 possible ways to get 6 tails in 9 flips.

Total outcome is 2^9 = 512

84/512 = 16%

Kluzty got 1/6 and it seemed really fast. Can you elaborate on the success failure thing you talked about?

NcR * Probability of success raised to the power of the number of successes*Probability of failure raised to the power of the number of failures.

Always works when there are only 2 possible outcomes, success or failure.

how do you know what the probability is out of...you basically have (1/2)^6 x (1/2)^3 = 1/256, now what???

osimsDDS said:

how do you know what the probability is out of...you basically have (1/2)^6 x (1/2)^3 = 1/256, now what???

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Then multiply by your NcR or in this case 9c6.

osimsDDS said:

how do you know what the probability is out of...you basically have (1/2)^6 x (1/2)^3 = 1/256, now what???

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Osims,how did u get 256!
Should be 512!

Glycogen said:

Osims,how did u get 256!
Should be 512!

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yea sorry i calculated it wrong in my head my bad your right its supposed to be 1/512

Then I guess you do 9!/6!(9-6)! = 84

so 84 x (1/512) = 84/512

Glycogen said:

Osims,how did u get 256!
Should be 512!

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Correct it should be 512, I didn't bother to check his numbers.

And so in this case if order would matter,we would use permutation one,right????
But the denominator would be the same??? 512????

Glycogen said:

And so in this case if order would matter,we would use permutation one,right????
But the denominator would be the same??? 512????

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I believe that is correct although i have never seen a question in my life that involved this formula and had permutations. They always have combinations.
Perhaps streetwolf has may know a drop more.