Question
Tue March 23, 2010 By:

Probability

Expert Reply
Tue March 23, 2010

(d) None of these.

The probabilities are binomial distribution.

Probability of getting 7 heads,

P(7) = n!(0.5)7(0.5)n-7/(7!(n-7)!) = n!(0.5)n/(7!(n-7)!)

Similarly, probability of getting 9 heads,

P(9) = n!(0.5)9(0.5)n-9/(9!(n-9)!) = n!(0.5)n/(9!(n-9)!)

We require, P(7) = P(9)

Hence we get

(9!(n-9)!) = (7!(n-7)!)

(n-7)! = (n-9)!x(n-8)x(n-7)

8x9 = (n-8)x(n-7) = n2 - 15n + 56

n2 - 15n + 56 = 72

n2 - 15n - 16 = 0

(n-16)(n+1) = 0

n = 16.

Regards,

Team,

TopperLearning.

 

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