G1.3 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 $c$.
$$
\begin{array}{l}
x=2.5 y \ldots \ldots \ldots \ldots .(1) \\
2 y=\frac{c}{100} \cdot x \ldots \ldots . .(2)
\end{array}
$$

Sub. (1) into (2): $2 y=\frac{c}{100} \cdot 2.5 y$
$$
c=80
$$