Teacher Li went to the store to buy a pen and paid 50 yuan. Get back 15 yuan, the price of each pen is 3.5 yuan, how many pens did Mr. Li buy? (Solution by equation).
李老师去商店买钢笔，付出50元．找回15元，每枝钢笔的售价是3.5元，李老师买了多少枝钢笔？(用方程解)．
<gadget id="calculator">50 - 15</gadget>
<output>35</output>

<gadget id="calculator">35 / 3.5</gadget>
<output>10</output>

<result>10</result>