Taotao reads a copy of "Harry Potter", the ratio of the number of pages read to the total number of pages in the first week is 2:5, if he reads another 20 pages, he has read half of the book. How many pages does this Harry Potter book have?
淘淘看一本《哈利波特》，第一周看的页数与总页数的比是2：5，如果他再看20页，就看了这本书的一半．这本《哈利波特》共有多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">20 / (1/10)</gadget>
<output>200</output>

<result>200</result>