A copy of "Happy Composition" is 19.9 yuan, and a copy of "Interesting Mathematics" is 22.8 yuan. Xiaohong bought a copy of "Happy Composition" and a copy of "Interesting Mathematics", and paid 100 yuan to the salesperson. How much should I get back?
一本《快乐作文》19.9元，一本《趣味数学》22.8元，小红买一本《快乐作文》和一本《趣味数学》，付给营业员100元，应找回多少元？
<gadget id="calculator">19.9 + 22.8</gadget>
<output>42.7</output>

<gadget id="calculator">100 - 42.7</gadget>
<output>57.3</output>

<result>57.3</result>