I4.3 If a number is selected from the whole numbers 1 to $m$, and if each number has an equal chance of being selected, the probability that the number is a factor of $m$ is $\frac{p}{39}$, find $p$.
Let $S$ be the sample space, $n(S)=78$
Favourable outcomes $=\{1,2,3,6,13,26,39,78\}$
$$
\begin{array}{l}
\frac{p}{39}=\frac{8}{78} \\
\Rightarrow p=4
\end{array}
$$