Pens are 12 yuan each, and ballpoint pens are 7 yuan each. Xiao Ming bought 6 pieces in total and spent 52 yuan. How many ballpoint pens did he buy?
钢笔每支12元，圆珠笔每支7元，小明共买了6支，花了52元，他买了几支圆珠笔？
<gadget id="calculator">6 * 12</gadget>
<output>72</output>

<gadget id="calculator">72 - 52</gadget>
<output>20</output>

<gadget id="calculator">12 - 7</gadget>
<output>5</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<result>4</result>