Xiaohong bought a pen with a unit price of 7.5 yuan. She paid 40 yuan and got back 2.5 yuan. How many pens did she buy?
小红买单价是7.5元的钢笔，付了40元钱，找回2.5元，她买了几枝钢笔？
<gadget id="calculator">40 - 2.5</gadget>
<output>37.5</output>

<gadget id="calculator">37.5 / 7.5</gadget>
<output>5</output>

<result>5</result>