probability

[tRNA]

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what is the probability of 6 tails out of 9 tosses of a fair coin?
answer 21/128
plz explain??

 

[mac60]

what is the probability of 6 tails out of 9 tosses of a fair coin?
answer 21/128
plz explain??

You need to get any combination of 6 tails and 3 heads, which brings this to the following permutation:

9!/6!3! = 84

Also, for a consecutive of nine throws, you're going to get 2^9 of possible occurrences.

P = 84/(2^9) = 21/128