When Xiao Ming reads a book, the ratio of the number of pages he has read to the number of pages he has not read is 2:3. If you read another 20 pages, you have read exactly 50% of the book. How many pages are there in this book?
小明看一本书，已看完的页数与未看完的页数的比是2：3．如果再看20页，就正好看了这本书的50%，这本书共有多少页？
<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>