Let $A=\{ x: x$ is a multiple of $3\}$ and $B=\{x:x$ is a multiple of $5\}.$ Then $A \cap B$ is given by ;

Given : $A=\{3,6,9,15\dots \}$ and $\mathrm{B}=\{5,10,15,20,\dots ..\}$

$\mathrm{A}\cap \mathrm{B}=\{\mathrm{x}:x$ is multiple of $3$ and 5 }

$\begin{array}{r}\Rightarrow \mathrm{A}\cap \mathrm{B}=\{\mathrm{x}:\mathrm{x}\text{ is multiple of }15\}\\ \Rightarrow \mathrm{A}\cap \mathrm{B}=\{15,30,45,\dots .\}\end{array}$