Multiple choice

A fair coin is tossed a fixed number of items. If the probability of getting seven heads is equal to that of getting nine heads, the probability of getting two heads is:

  1. 15/28

  2. 2/15

  3. 15/213

  4. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For a binomial distribution, P(X=k) = C(n,k) * p^k * (1-p)^(n-k). If p=0.5, P(X=7) = P(X=9) implies C(n,7) = C(n,9), so n = 7+9 = 16. The probability of 2 heads is C(16,2) * (0.5)^16 = (16*15/2) / 2^16 = 120 / 2^16 = 15 / 2^13.

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