A fair coin is flipped 7 times. What is the probability that at least 5 of the flips come up heads?
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The sum of these probabilities gives us the answer
P(5 heads) = C(7,5) * (.5)^5 * (.5)^2 +
P(6 heads) = C(7,6) * (.5)^6 * (.5)^1 +
P(7 heads) = (.5)^7
= 29 / 128 = about 22.66%
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