Reading a book, Xiaohong read 10% for the first time, 20 pages for the second time, and left 50% of the book, how many pages does this book have?
看一本书，小红第一次看了10%，第二次看了20页，还剩全书的50%没看，这本书一共有多少页？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/10) - (1/2)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">20 / (2/5)</gadget>
<output>50</output>

<result>50</result>