I3.4 $x, y$ are real numbers. If $x+y=c-195$ and $d$ is the maximum value of $x y$, find $d$.
Reference: 1988 FI4.3
$$
\begin{array}{l}
x+y=4 \\
\Rightarrow y=4-x \\
x y=x(4-x)=-(x-2)^{2}+4 \leq d \\
\Rightarrow d=4
\end{array}
$$