G9.1 7 oranges and 5 apples cost $\$ 13 . \quad 3$ oranges and 4 apples cost $\$ 8 . \quad 37$ oranges and 45 apples cost $\$ C$. Find $C$.
Let the cost of an orange be $\$ x$ and the cost of an apple be $\$ y$.
$$
\begin{array}{l}
7 x+5 y=13 \cdots \cdots \cdot(1) \\
3 x+4 y=8 \cdots \cdots \cdot(2) \\
4(1)-5(2): 13 x=12 \\
\Rightarrow x=\frac{12}{13} \\
7(2)-3(1): 13 y=17 \\
\Rightarrow y=\frac{17}{13} \\
C=37 x+45 y=\frac{12 \times 37+17 \times 45}{13}=93
\end{array}
$$