Car model 10 yuan/pen 2.5 yuan/piece. Dongdong bought two small car models and several pens for 30 yuan. Do you know how many pens he bought?
小车模型10元/个钢笔2.5元/支．东东买了两个小车模型和几支钢笔共用去30元，你知道他买了几支钢笔吗？
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<gadget id="calculator">10 / 2.5</gadget>
<output>4</output>

<result>4</result>