Let A ={1,2,3, …, 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}.…

Let $\displaystyle \mathrm{A}=\{1,2,3, \ldots, 20\}$. Let $\displaystyle \mathrm{R}_1$ and $\displaystyle \mathrm{R}_2$ two relation on A such that $\displaystyle R_1=\{(a, b): b$ is divisible by $\displaystyle a\}$ $\displaystyle R_2=\{(a, b): a$ is an integral multiple of $\displaystyle b\}$. Then, number of elements in $\displaystyle \mathrm{R}_1-\mathrm{R}_2$ is equal to $\displaystyle \_\_\_\_$.