答案：C
該六個數的平均值
該六個數的平均值
$\begin{array}{cl}
= & \dfrac{1}{6}(20a+3 + 20a + 5 + \ldots + 20a + 17) \\
= & 20a + 10
\end{array}$
由此，該六個數的方差
$\begin{array}{cl}
= & \dfrac{1}{6}\{[(20a+3) – (20a+10)]^2 + [(20a+5) – (20a+10)]^2 + \ldots + [(20a+17) – (20a + 10)]^2\} \\
= & 25
\end{array}$
