A pen is 4.5 yuan, Xiaohong gives 20 yuan to the salesperson's aunt, and gets back 6.5 yuan, how many pens did she buy?
一支钢笔4.5元，小红给售货员阿姨20元，找回6.5元，她买了几支钢笔？
<gadget id="calculator">20 - 6.5</gadget>
<output>13.5</output>

<gadget id="calculator">13.5 / 4.5</gadget>
<output>3</output>

<result>3</result>