If 50% of (x - y) = 30% of (x + y), then what percent of x is y ?

Answer: Option A

$$\begin{array}{rl}& 50\%\text{ of }\left(x-y\right)=30\%\text{ of }\left(x+y\right)\\ & \Rightarrow 5\left(x-y\right)=3\left(x+y\right)\\ & \Rightarrow 5x-5y=3x+3y\\ & \Rightarrow 2x=8y\\ & \Rightarrow y=\frac{x}{4}\end{array}$$
∴ Required percentage :
$$\begin{array}{rl}& =\left(\frac{y}{x}\times 100\right)\%\\ & =\left(\frac{x}{4}\times \frac{1}{x}\times 100\right)\%\\ & =25\%\end{array}$$