答案：(a) $\$300$ (b) $\$400$
- 該書本的售價
$\begin{array}{cl}
= & 250 \times (1+20\%) \\
= & \$ 300
\end{array}$ - 該書本的標價
$\begin{array}{cl}
= & 300 \div (1 – 25\%) \\
= & \$ 400
\end{array}$
