Xiao Ming reads a book and has already read (2/5). If he reads another 30 pages, he can finish half of the book. How many pages does this book have?
小明看一本书，已经看了(2/5)，再看30页正好可以看完这本书的一半，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">30 / (1/10)</gadget>
<output>300</output>

<result>300</result>