Question

An integer is chosen from the first twenty natural numbers. What is the probability that it is a prime number?

Answer 1

An integer is chosen from first twenty natural numbers.

Thus, $S=\{1,2,3,4,....20\}$
$\Rightarrow n\left(S\right)=20$
Let $A$ be event of getting prime number.
$\Rightarrow A=\{2,3,5,7,11,13,17,19\}$
$\Rightarrow n\left(A\right)=8$
$\Rightarrow P\left(A\right)=\frac{n\left(A\right)}{n\left(S\right)}=\frac{8}{20}=\frac{2}{5}$