math question please help!

[Questions]

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What is the probabilty of getting 6 tails out of 9 coin tosses?

 

[Moabmad]

2 Possible outcomes --> H or T. Therefore total outcomes = 2^9 = 512

Getting 6 tails out of 9 tosses --> Use combination equation.

n!/(n-r)!r! --> 9!/3!6! --> 168 ( total ways of getting 6 tails (or heads if that was the question) out of the 512 total outcomes)

Probablility = 168/512 = 0.328 or 1/3

Hope the computations is right, I used the stupid windows calc.

 

[Impulse155]

2 Possible outcomes --> H or T. Therefore total outcomes = 2^9 = 512

Getting 6 tails out of 9 tosses --> Use combination equation.

n!/(n-r)!r! --> 9!/3!6! --> 168 ( total ways of getting 6 tails (or heads if that was the question) out of the 512 total outcomes)

Probablility = 168/512 = 0.328 or 1/3

Hope the computations is right, I used the stupid windows calc.

When you do factorials isnt 9!/3!6! the same as
(9*8*7*6*5*4*3*2)/(3*2)(6*5*4*3*2)
which is
362880/720*6
362880/4680
=84
So 84/512
=21/128 or .164

 

[Moabmad]

When you do factorials isnt 9!/3!6! the same as
(9*8*7*6*5*4*3*2)/(3*2)(6*5*4*3*2)
which is
362880/720*6
362880/4680
=84
So 84/512
=21/128 or .164

Yup. Checked it on my calc. Thanks. 👍😳

 

[Questions]

thank you so much!

 

[sfoksn]

we dont' have to be able to do this for QR, do we?

how do we do this w/o calculators??

 

[Moabmad]

we dont' have to be able to do this for QR, do we?

how do we do this w/o calculators??

You are expected to be able to do this on the QR, maybe not to this difficulty. I did have quite a number of combinations and permutations on my QR section. I memorized factorials so that I wasnt wasting time multiplying them out. e.g. 3! = 6, 5!=120, 7! = 5,040

Also, if you realize with factorials

7!/5! = 7*6*5*4*3*2*1 / 5*4*3*2*1 everything cancels except

7*6 it makes it alot faster.

On my QR section, I would say typically the questions contained numbers that could reasonably be computed in your head. However, there were questions that all I could do is quickly and vaguely estimate the answer.