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 

A = {x : x is a multiple of 3}

$\Rightarrow \mathrm{A}=\{0,3,6,9,12,15\dots ..\}$

B = {x : x is a multiple of 5}

$\Rightarrow \mathrm{B}=\{0,5,10,15,20,25,30\dots \}$

$\mathrm{A}\cap \mathrm{B}=\{15,30,45\dots \dots \}$