When Xiaohua reads a book, the ratio of pages he has read to that he has not read is 2:5, and after reading 50 pages, the ratio of pages he has read to pages he has not read is 3:4, so how many pages are left to be read? ．
小华看一本书，已看的和未看的页数比是2：5，再看50页，已看的和未看的页数比是3：4，那么还有多少页未看完．
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">2 + 5</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">(3/7) - (2/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">50 / (1/7)</gadget>
<output>350</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">350 * (4/7)</gadget>
<output>200</output>

<result>200</result>