A fountain pen costs 8.5 yuan, and 3 ballpoint pens cost 16.5 yuan. How much is a ballpoint pen cheaper than a fountain pen?
一支钢笔售价8.5元，3支圆珠笔售价16.5元．一支圆珠笔比一支钢笔便宜多少钱？
<gadget id="calculator">16.5 / 3</gadget>
<output>5.5</output>

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

<result>3</result>