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

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