SG. 3 Let $x, y$ be non-zero real numbers. If $x$ is $250 \%$ of $y$ and $2 y$ is $c \%$ of $x$, find the value of $c$.
$\begin{array}{l}x=2.5 y \ldots \ldots \ldots \ldots . . . .1) \\ 2 y=\frac{c}{100} \cdot x \ldots \ldots . .(2) \\ \text { sub. (1) into (2): } 2 y=\frac{c}{100} \cdot 2.5 y \\ c=80\end{array}$