Question

From $21$ tickets numbered $1,2,3,....21$, one ticket is drawn at random. The probability that the ticket drawn has a number divisible by $3$ is $\frac{k}{21}$. Find the value of $k$.

Answer 1

Total possibilities $=n\left(S\right)=21$
Numbers divisible by $3$ are $3,6,9,12,15,18$ and $21$ $\Rightarrow$ $n\left(E\right)=7$
The probability$=\frac{7}{21}$
$\Rightarrow k=7$