The school bought a batch of books, including 310 story books, 240 literature and art books, and the rest were science and technology books. It is known that science and technology books accounted for 45% of the total number of books. How many science and technology books did they buy?
学校买一批书，其中有故事书310本，文艺书240本，其余是科技书，已知科技书占这批书总数的45%，买来科技书多少本？
<gadget id="calculator">310 + 240</gadget>
<output>550</output>

<gadget id="calculator">45 / 100</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">1 - (9/20)</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">550 / (11/20)</gadget>
<output>1_000</output>

<gadget id="calculator">1_000 * (9/20)</gadget>
<output>450</output>

<result>450</result>