Question

In order to get a head at least once probability \(\geq 0.9\), the minimum number of time a unbiased coin needs to be tossed is
(A) 3
(B) 4
(C) 5
(D) 6

Answer 1

Ans: (B)
Hint : Let \(x=\) no. of heads appear in \(n\) tossed
$$
X-\operatorname{Bin}\left(n, \frac{1}{2}\right)
$$
Now, \(P(x \geq 1)=1-P(x=0)=1-\frac{1}{2^{n}} \geq 0.9 \Rightarrow \frac{1}{2^{n}} \leq \frac{1}{10} \Rightarrow n \geq 4\) \(\therefore\) minimum number of tosses \(=4\)

Answer 2

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Answer 3

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