答案：(a) $\$320$ (b) $28\%$
- 該花瓶的售價
$\begin{array}{cl}
= & \$400 \times (1-20\%) \\
= & \$320
\end{array}$ - 該花瓶的成本
$\begin{array}{cl}
= & \$320 – \$70 \\
= & \$250
\end{array}$所以，盈利率
$\begin{array}{cl}
= & \dfrac{70}{250} \times 100\% \\
= & 28\%
\end{array}$
