Question

A person buys a lottery ticket in 50 lotteries, in each of which his chance of winning a prize is `1/100`  What is the probability that he will win a prize at least twice.

Answer 1

Let X denote the number of times the person wins the lottery. 
Then, X follows a binomial distribution with n = 50.

\[\text{ Let p be the probability of winning a prize }.\]

\[ \therefore p = \frac{1}{100}, q = 1 - \frac{1}{100} = \frac{99}{100}\]

\[\text{ Hence, the distribution is given by } \]

\[P(X = r) =^{50}{}{C}_r \left( \frac{1}{100} \right)^r \left( \frac{99}{100} \right)^{50 - r} , r = 0, 1, 2 . . . 50\]

\[ P(\text{ winning at lease twice}) = P(X\geq 2)\]

\[ = 1 - P(X = 0) - P(X = 1)\]

\[ = 1 - \left( \frac{99}{100} \right)^{50} - ^{50}{}{C}_1 \times \frac{1}{100} \times \left( \frac{99}{100} \right)^{49} \]

\[ = 1 - \frac{{99}^{49} \times 149}{{100}^{50}}\]

Hence, the probability of winning the prize at least twice \[ = 1 - \frac{{99}^{49} \times 149}{{100}^{50}}\]