When Xiaohua reads "Historical Stories", the ratio of the number of pages he read on the first day to the total number of pages in the book is 2:5. If he reads another 18 pages, he can read half of the book. How many pages does this book have?
小华看《历史故事》书，第一天看的页数与全书总页数的比是2：5，如果再看18页，就可看完全书的一半．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">18 / (1/10)</gadget>
<output>180</output>

<result>180</result>