Science and technology books in the school library account for 40% of the total, and story books account for 35% of the total. There are 210 more science and technology books than story books. How many books are there in the school library?
学校图书室科技书占总数的40%，故事书占总数的35%，科技书比故事书多210本，学校图书室共有多少本书？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">35 / 100</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">(2/5) - (7/20)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">210 / (1/20)</gadget>
<output>4_200</output>

<result>4_200</result>