优惠券折算示例
<p>优惠券:满 12 减 3</p>
<table>
<thead>
<tr>
<th>商品</th>
<th>单价</th>
<th>数量</th>
</tr>
</thead>
<tbody>
<tr>
<td>a</td>
<td>3</td>
<td>2</td>
</tr>
<tr>
<td>b</td>
<td>4</td>
<td>1</td>
</tr>
<tr>
<td>c</td>
<td>7</td>
<td>1</td>
</tr>
</tbody>
</table>
<p>商品总金额 = 3 * 2 + 4 + 7 = 17
总支付金额 = 17 - 3 = 14</p>
<p>每个商品折算后的实际支付金额:
a = 14 * 6 / 17 ≈ 4.94
b = 14 * 4 / 17 ≈ 3.29
c = 14 - 4.94 - 3.29 = 5.77 = 14 * 7 / 17 ≈ 5.76</p>
<blockquote>
<p>计算最后一个商品时,如果小数<strong>除不尽</strong>或之前的商品小数<strong>除不尽</strong>,会造成总支付金额加和与实际总支付金额对不上的问题(即4.94+3.29+5.76 = 13.99 <> 14),因此最后一个商品需要用<strong>减法</strong></p>
</blockquote>