Xiao Ming went to buy a pen with a unit price of 3.5 yuan, paid 20 yuan, and got back 2.5 yuan. How many pens did she buy?
小明去买单价为3.5元的钢笔，付了20元钱，找回2.5元，她买了几枝钢笔？
<gadget id="calculator">20 - 2.5</gadget>
<output>17.5</output>

<gadget id="calculator">17.5 / 3.5</gadget>
<output>5</output>

<result>5</result>