how many time

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how many time

by maihuna » Sat Aug 15, 2009 1:52 am
How many times must a man toss a fair coin so that the probability of having
at least one head is more than 90%?
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Re: how many time

by mohitsharda » Sat Aug 15, 2009 2:18 am
maihuna wrote:How many times must a man toss a fair coin so that the probability of having
at least one head is more than 90%?
With a fair coin, probability of getting heads = Prob (tails) = .5

Prob (at least 1 head in n tries) = 1 - Prob ( all tails in n tries)
= 1 - (.5)^n

According to question

=> 1- (.5)^n >= .9
=> .5^n =< .1

=> (1/2)^n =< .1
=> 1 =< .1* (2^n)
=> 10 =< 2^n

So, n = 4 ... minimum integral value for which the above equation is satisfied.
MS