Mr. Li brought 500 yuan to the bookstore to buy books. It is known that a fairy tale book is 25 yuan, and he bought 18 fairy tale books. How much should the bookstore give back to Mr. Li?
李老师带500元钱去书店买书，已知一本童话故事书25元，买了18本童话故事书，书店应找回李老师多少钱？
<gadget id="calculator">25 * 18</gadget>
<output>450</output>

<gadget id="calculator">500 - 450</gadget>
<output>50</output>

<result>50</result>