Explanation

(b)
Let $X$ be the number of heads getting in $n$ tossed. Therefore, $X$ follows binomial distribution with parameters
$n,P = \frac{1}{2},q = \frac{1}{2}$
Since, $P(X \geq 1) \geq 0.8$ [given]
$\therefore 1 - P(X = 0) \geq 0.8$
$\Rightarrow \ P(X = 0) \leq 0.2$
$\Rightarrow {\ ^{n}C}_{0}\left( \frac{1}{2} \right)^{0}\left( \frac{1}{2} \right)^{n} \leq 0.2$
$\Rightarrow \ \frac{1}{2^{n}} \leq \frac{1}{5}\ \Rightarrow \ 2^{n} \geq 5$
Hence, least value of $n$ is 3