Question 1124876
2^9 = 512 outcomes
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the binomial probability formula is
:
Probability (P) ( k successes in n trials ) = nCk * p^k * (1-p)^(n-k), where nCk =  n!/k!(n-k)!, p is probability of success
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if k = 3, then
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P (3 heads in 9 trials) = 9C3 * (1/2)^3 * (1-(1/2))^(9-3) =
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9!/3!(9-3)! * (1/8) * (1/64) =
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(9*8*7/(3*2)) * (1/512) =
:
84/512
:
There are 84 outcomes where heads comes up three times
:
P of HHHTTTTTT = (1/2)^3 * (1/2)^6 = (1/8) * (1/64) = 1/512
:
P heads comes up three times is 84/512 = 21/128
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