In a book, the ratio of the number of pages you have read to the number of pages you have not read is 1:3. If you read another 150 pages, you have read half of the book. How many pages does this book have?
一本书，已看和未看的页数的比是1：3，如果再看150页，正好看了全书的一半．这本书共有多少页？
<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">150 / (1/4)</gadget>
<output>600</output>

<result>600</result>