$\text{ If }S=\{x\mid x\text{ is a positive multiple of }3\text{ and less than }100\}\text{ and }P=\{x\mid x\text{ is a prime number less than }20\}\text{ . Then }n\left(S\right)+n\left(P\right)$

$\mathrm{S}=\{x\mid x\text{ is a positive multiple of }3\text{ and less than }100\}$

$\begin{array}{r}=\{3,6,9,\dots ,99\}\\ \therefore n\left(S\right)=33\end{array}$

$\begin{array}{r}\text{ and }P=\{x\mid x\text{ is a prime number less than }20\}\\ =\{2,3,5,7,11,13,17,19\}\end{array}$

$\begin{array}{l}\therefore n\left(P\right)=8\\ n\left(S\right)+n\left(P\right)=33+8=41\end{array}$