Solution

Let $H$ and  $T$ be the event that head and tail is obtained on toss, respectively.

$$T

$$H

Let $n$ be the number of tosses.

$$$$P(H)=P(T)=0.5

Let $A$ be the event that at least one head comes up.

We want to find $n \in \mathbb{N}$ such that $P(A) \ge 0.9$

$$$$\begin{align*} P(A)&\ge 0.9\\ 1- (0.5)^n &\ge 0.9\\ (0.5)^n&\le0.1\\ n \ge 4 \end{align*}

Thus, if coin is tossed 4 times or more, the probability of getting at least one head is equal to or greater than 0.9

Answer

4

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