Xiao Ming buys a fountain pen and a ballpoint pen, and pays 8.5 yuan in total. If he uses one pen to exchange two ballpoint pens, he has to pay the salesperson 0.5 yuan. How much is a pen?
小明买钢笔和圆珠笔各一支，共付8.5元，如果用一支钢笔换2支圆珠笔还要付给营业员0.5元，一支钢笔多少元？
<gadget id="calculator">8.5 + 0.5</gadget>
<output>9</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<gadget id="calculator">8.5 - 3</gadget>
<output>5.5</output>

<result>5.5</result>