Mr. Zhang bought a writing brush for 28.5 yuan and a set of books for 42.8 yuan in the supermarket. He paid 100 yuan. How much should he get back?
张老师在超市买了一枝28.5元的毛笔和一套42.8元的书，他付了100元，应找回多少元？
<gadget id="calculator">28.5 + 42.8</gadget>
<output>71.3</output>

<gadget id="calculator">100 - 71.3</gadget>
<output>28.7</output>

<result>28.7</result>