smc

We see that $x$ is multiplied by $\frac{100+p}{100}$ when it is increased by $p$%. Therefore, $y$ is multiplied by $\frac{100}{100+p}$, and so it is decreased by $\frac{p}{100+p}$ times itself. Therefore, it is decreased by $\frac{100p}{100+p}$%, and so the answer is $\boxed{\frac{100p}{100+p}\%}$.