In the library, science and technology books account for 20% of the book collection, and story books account for 45% of the book collection. There are 500 fewer science and technology books than story books. How many books are there in the library?
图书室里科技书占藏书量的20%，故事书占藏书量的45%，科技书比故事书少500本．图书室共有藏书多少本？
<gadget id="calculator">45 / 100</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(9/20) - (1/5)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">500 / (1/4)</gadget>
<output>2_000</output>

<result>2_000</result>