Xiao Ming bought a pen with a unit price of 4.5 yuan, paid 20 yuan, and got back 6.5 yuan. How many pens did he buy?
小明买了单价是4.5元的钢笔，付20元，找回6.5元，他买了几枝钢笔？
<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>