A certain courier company already has some shipments, but there are still shipments. The company decides to use the express car to distribute the express to the customer (loading time is not counted). If 9 vehicles are used to deliver the goods, the shipment can be completed within 12 hours. If 8 vehicles are used to deliver the goods, it can be shipped within 16 hours. The courier company only used 6 vehicles to deliver the goods at the beginning, and added several vehicles after three hours. After another 5 hours, the shipment will be finished, so how many vehicles should be added later?
某快递公司已存在部分快件，但仍有快件不断运来．公司决定用快递专车将快件分给客户(装车时间不计)若用9辆车发货，12小时可运完．若用8辆车发货，16小时可运完．快递公司开始只用了6辆车发货，三小时后增加若干辆车．再经过5小时就运完了，那么后来增加的车辆数应该是多少辆？
<gadget id="calculator">9 * 12</gadget>
<output>108</output>

<gadget id="calculator">8 * 16</gadget>
<output>128</output>

<gadget id="calculator">128 - 108</gadget>
<output>20</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">12 * 5</gadget>
<output>60</output>

<gadget id="calculator">108 - 60</gadget>
<output>48</output>

<gadget id="calculator">5 * 5</gadget>
<output>25</output>

<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">15 - 18 - 30</gadget>
<output>-33</output>

<gadget id="calculator">48 + 25 + (-33)</gadget>
<output>40</output>

<gadget id="calculator">40 / 5</gadget>
<output>8</output>

<result>8</result>