The minimum number of times one has to toss a fair coin

Probability of observing at least one head out of $n$ tosses

$=1-\left(\frac{1}{2}\right)^{\mathrm{n}} \geq 0.9$

$\Rightarrow\left(\frac{1}{2}\right)^{\mathrm{n}} \leq 0.1$

$\Rightarrow n \geq 4$

$\Rightarrow$ minimum number of tosses $=4$