8. It is given that $\sqrt{a}-\sqrt{b}=20$, where $a$ and $b$ are real numbers. Find the maximum possible value of $a-5 b$.
8. Answer 500
$$
\begin{array}{l}
\sqrt{a}=\sqrt{b}+20 \\
a=b+400+40 \sqrt{b} \\
a-5 b=400+40 \sqrt{b}-4 b \\
a-5 b=400-4(\sqrt{b}-5)^{2}+100 \\
a-5 b \leq 500
\end{array}
$$