Mr. Li bought 25 notebooks, each notebook is 3.4 yuan, and paid 100 yuan, how much should I get back?
李老师买了25本笔记本，每本笔记本3.4元，付100元，应找回多少钱？
<gadget id="calculator">3.4 * 25</gadget>
<output>85</output>

<gadget id="calculator">100 - 85</gadget>
<output>15</output>

<result>15</result>