Mr. Li bought 8 pens and rewarded the students. Each pen was X yuan, and he paid 50 yuan. How much should he get back?
李老师买了8支钢笔奖励给学生，每支X元，付出50元，应找回多少元．
<gadget id="calculator">50 - 8</gadget>
<output>42</output>

<result>42</result>