A) 2
B) 3
C) 4
D) 5
Correct Answer: B
Solution :
Let X be the number of heads getting. X follows binomial distribution with parameters n, \[\operatorname{p}= 1/2.\] Given that \[\operatorname{P}(X\ge 1)\ge 0.8\] \[\Rightarrow \,\,\,1-P(X=0)\ge \,\,.0.8\,\,\Rightarrow \,\,P(X=0)\le 0.2\] \[\Rightarrow \,\,{{\,}^{n}}{{C}_{0}}{{(1/2)}^{n}}\le 0.2\,\,\Rightarrow \,\,\frac{1}{{{2}^{n}}}\le \frac{1}{5}\Rightarrow {{2}^{n}}\ge 5\] \[\therefore \] the least value of n is 3.
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