Math
posted by Angie .
If four coins are flipped at once, what is the probability that exactly 3 tails will come up?
a. 1/16
b. 1/8
c. 1/4
d. 1/2
The answer is c, but what do you do to get that?

Binomial distribution
p(n,k) = C(n,k) p^(k) (1p)^(nk)
here n = 4, k = 3, p =.5
C(4,3) = 4!/[3!*1!] = 4 or use Pascal triangle or table of binomial coefs
so
4 (.5)^3*.5
= .25 sure enough c
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