Question

A bag contains $20$ balls numbered from $1$ to $20$. One ball is drawn at random from the bag. What is the probability that the ball drawn is marked with a number which is multiple of $3$ or $4$?

Answer 1

Number of balls $=20$
$\therefore n\left(S\right)=20$
Number of balls marked with multiple of $3$ or $4$ is $3,6,9,12,15,18,4,8,16,20$
$\therefore n\left(A\right)=10$
The required probability $P\left(A\right)=\frac{n\left(A\right)}{n\left(S\right)}=\frac{10}{20}=\frac{1}{2}$