There are a total of 21,000 science and art books in the school library, among which science and technology books account for (5/7), and some science and technology books were bought later, so that literature and art books accounted for (2/9) of science and technology books, and how many science and technology books were bought later Book?
学校图书馆科技书和文艺书共21000本，其中科技书占(5/7)，后来又买了一些科技书，这样文艺书占科技书的(2/9)，后来又买了多少本科技书？
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">1 + (5/7)</gadget>
<output>12/7 = around 1.714286</output>

<gadget id="calculator">21_000 / (12/7)</gadget>
<output>12_250</output>

<gadget id="calculator">21_000 - 12_250</gadget>
<output>8_750</output>

<gadget id="calculator">2 / 9</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">8_750 / (2/9)</gadget>
<output>39_375</output>

<gadget id="calculator">39_375 - 12_250</gadget>
<output>27_125</output>

<result>27_125</result>