Teacher Wang bought 3 notebooks and some pens, and paid a total of 77 yuan, 9.5 yuan for each pen, and 3.5 yuan for each notebook. How many pens did Teacher Wang buy?
王老师买了3本笔记本和一些钢笔，共付了77元，每支钢笔9.5元，每本笔记本3.5元，王老师买了几支钢笔？
<gadget id="calculator">3.5 * 3</gadget>
<output>10.5</output>

<gadget id="calculator">77 - 10.5</gadget>
<output>66.5</output>

<gadget id="calculator">66.5 / 9.5</gadget>
<output>7</output>

<result>7</result>