I1.4 Given that the difference between two 3-digit numbers $\overline{x y z}$ and $\overline{z y x}$ is $700-c$, where $x>z$. If $d$ is the greatest value of $x+z$, find the value of $d$.
$$
\begin{array}{l}
\overline{x y z}-\overline{z y x}=700-c \\
100 x+10 y+z-(100 z+10 y+x)=700-7 \\
99 x-99 z=693 \\
x-z=7
\end{array}
$$

Possible answers: $x=8, z=1$ or $x=9, z=2$ $d$ is the greatest value of $x+z=9+2=11$