Li Xin reads a story book, and the ratio of the number of pages she has read to the number of unread pages is 1:3. If she is reading 50 pages, and she has read exactly half of the book, how many pages does the book have?
李欣读一本故事书，已读的页数和未读的比是1：3，如果她在读50页，已读的正好占全书的一半，这本书共多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</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">(1/2) - (1/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">50 / (1/4)</gadget>
<output>200</output>

<result>200</result>