Xiaogang reads a book. He reads 30% of the whole book on the first day, and reads 15 pages less on the second day than on the first day. At this time, he still has half of the book. How many pages does this book have?
小刚读一本书，第一天读了全书的30%，第二天比第一天少读15页，这时还有一半没有读，这本书共有多少页？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(3/10) - (1/2)</gadget>
<output>-1/5 = around -0.2</output>

<gadget id="calculator">(3/10) + (-1/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">15 / (1/10)</gadget>
<output>150</output>

<result>150</result>