In a seaport freight yard, there are foreign ships unloading the goods one after another, and the goods are transported away by cars one after another. If you use 9 cars, you can clear the field in 12 hours; if you use 8 cars, you can clear the field in 16 hours. At the beginning, only 3 cars were used in the field, and some cars were added in 10 hours, and the field was cleared after 4 hours, so how many cars should be added later?
某海港货场不断有外洋轮船卸下货来，又不断用汽车将货物运走．如果用9辆车，12小时可以清场；如果用8辆车，16小时也可以清场．该场开始只用3辆车，10小时候增加了若干辆车，再过4小时就已清场，那么后来增加的车数应是多少．
<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">2 * 10</gadget>
<output>20</output>

<gadget id="calculator">48 + 20</gadget>
<output>68</output>

<gadget id="calculator">68 / 4</gadget>
<output>17</output>

<gadget id="calculator">17 + 2</gadget>
<output>19</output>

<result>19</result>