When Xiao Ming reads a story book, the ratio of the number of pages he has read to the number of unread pages is 1:3. If he reads another 300 pages, the ratio of the number of pages he has read to the number of unread pages is 1:2. How many pages does this book have?
小明读一本故事书，已读的页数和未读的页数的比是1：3，如果再读300页，则已读的页数和未读的页数的比是1：2，这本书有多少页？
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/3) - (1/4)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">300 / (1/12)</gadget>
<output>3_600</output>

<result>3_600</result>