Xiao Xing reads an extra-curricular book. He reads 20% of the book on the first day and 15 pages on the second day. He read half of the book in two days. How many pages are there in this book?
小星看一本课外书，第一天看了全书的20%，第二天看了15页，两天一共看了这本书的一半．这本书一共多少页？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">15 / (3/10)</gadget>
<output>50</output>

<result>50</result>