When Xiao Ming reads a story book, the ratio of the number of pages he read to the total number of pages on the first day is 3:7. If he reads another 15 pages, it is exactly half of the book. How many pages does this book have?
小明看一本故事书，第一天看的页数与总页数的比是3：7，如果再看15页，正好是这本书的一半，这本书有多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(1/2) - (3/7)</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">15 / (1/14)</gadget>
<output>210</output>

<result>210</result>