When Xiao Ming reads a story book, the ratio of the number of pages he has read to the number of pages he has not read is 4:5. If he reads 15 more pages, he has read half of the book. How many pages are there in this book?
小明看一本故事书，已看的页数与未看页数的比是4：5，再看15页，就看了这本书的一半．这本书一共多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">(1/2) - (4/9)</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">15 / (1/18)</gadget>
<output>270</output>

<result>270</result>