Posted by **Angie** on Saturday, June 13, 2009 at 5:53pm.

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?

- Math -
**Damon**, Saturday, June 13, 2009 at 6:12pm
Binomial distribution

p(n,k) = C(n,k) p^(k) (1-p)^(n-k)

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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