Sets And Relations

Let $A=\{1,2,3, \ldots, 20\}$. Let $R_1$ and $R_2$ two relation on $A$ such that


$R_1=\{(a, b): b$ is divisible by $a\}$


$R_2=\{(a, b): a$ is an integral multiple of $b\}$.


Then, number of elements in $R_1-R_2$ is equal to _____________.