$x\% \text{ of } y$
=$\dfrac{x}{100} \times y$
=$\dfrac{xy}{100}$
$y\% \text{ of } x$
=$\dfrac{y}{100} \times x$
=$\dfrac{xy}{100}$
∴ $x\% \text{ of } y$ + $y\% \text{ of } x$
= $\dfrac{xy}{100}+\dfrac{xy}{100}$
= $\dfrac{2xy}{100}$
=$2\% \text{ of }xy$