Question

In order to get at least once a head with probability $\ge 0.9$, the number of times a coin needs to be tossed is

• A. $3$
• B. $4$
• C. $5$
• D. None of these

Answer 1

Answer: (B)

$4$

Since the probability of getting at least one head in $n$ tosses $=1-{\left(\frac{1}{2}\right)}^n$.
Therefore $1-{\left(\frac{1}{2}\right)}^n\ge 0.9\Rightarrow {\left(\frac{1}{2}\right)}^n\le 0.1$
$\Rightarrow 2^n\ge 10\Rightarrow n\ge 4$
Hence, the least value of $n$ is $4$.