There are 450 scientific and literary books, among which scientific books account for 2/9 of the total, and now I have bought some technical books, which now account for 2/7 of the total. How many technical books did you buy?
科技书和文艺书共450本，其中科技书占总数的(2/9)，现在又买了一些科技书，此时科技书占总数的(2/7)．又买来多少本科技书？
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">450 * (2/7)</gadget>
<output>900/7 = around 128.571429</output>

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

<gadget id="calculator">450 * (2/9)</gadget>
<output>100</output>

<gadget id="calculator">(900/7) - 100</gadget>
<output>200/7 = around 28.571429</output>

<gadget id="calculator">1 - (2/7)</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(200/7) / (5/7)</gadget>
<output>40</output>

<result>40</result>