Explanation

(b)
The probability of getting head at least once in $n$ times
$= 1 - P$(None of the trial getting head)
$= 1 - \left( \frac{1}{2} \right)^{n}$
Given $1 - \left( \frac{1}{2} \right)^{n} > 0.8 \Rightarrow \left( \frac{1}{2} \right)^{n} < 0.2$
$\Rightarrow 2^{n} > \frac{1}{0.2}\ \Rightarrow 2^{n} > 5$
Hence, least value of n is 3