Mr. Wang reads a book. On the first day, he reads (2/5) of the whole book. On the second day, he reads 15 pages. At this time, he just read half of the whole book. How many pages does this book have?
王老师看一本书，第一天看了全书的(2/5)，第二天看了15页，这时刚好看了全书的一半，这本书一共有多少页？
<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">15 / (1/10)</gadget>
<output>150</output>

<result>150</result>