Given, \[\mathbf{S}=\left\{ \mathbf{1},\text{ }\mathbf{2},\text{ }\mathbf{3},\text{ }\ldots .,~n \right\}\]
So, \[P\left( r\text{ }\le \text{ }p/s\text{ }\le \text{ }p \right)\text{ }=\text{ }P(P\cap S)/\text{ }P\left( S \right)\]
= \[p\text{ }\text{ }1/n\text{ }\times \text{ }n/\left( n\text{ }\text{ }1 \right)\]
= \[\left( p\text{ }\text{ }1 \right)/\left( n\text{ }\text{ }1 \right)\]
Therefore, the required probability is \[\left( p\text{ }\text{ }1 \right)/\left( n\text{ }\text{ }1 \right)\].