The school bought a batch of books, among which there are 400 more science books than story books, and the number of science books is three times that of story books. How many technical books are there? (solution by equation)
学校买来一批图书，其中科技书比故事书多400本，科技书的本书是故事书的3倍．科技书有多少本？(用方程解)
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">400 / 2</gadget>
<output>200</output>

<gadget id="calculator">200 * 3</gadget>
<output>600</output>

<result>600</result>