The minimum number of times one has to toss

Let, $p$ is probability for getting head and is probability for getting tail.

$p=P(H)=\frac{1}{2}, q=1-p=\frac{1}{2}$

$P(x \geq 1) \geq \frac{9}{10} \Rightarrow 1-P(x=0) \geq \frac{9}{10}$

$1-{ }^{n} C_{0}\left(\frac{1}{2}\right)^{n} \geq \frac{9}{10} \Rightarrow \frac{1}{2^{n}} \leq 1-\frac{9}{10} \Rightarrow \frac{1}{2^{n}} \leq \frac{1}{10}$

$2^{n} \geq 10 \Rightarrow n \geq 4 \Rightarrow n_{\min }=4$