Hao Wu reads a novel. After 3 days of reading, he finds that the ratio of the number of pages he has read to the number of pages left is 4:5. If he reads another 25 pages, he will read half of it. How many pages does this book have?
皓午看一本小说，看了3天后他发现已经看的页数与还剩的页数比是4：5，如果再看25页就正好看了一半，这本书有多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">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">25 / (1/18)</gadget>
<output>450</output>

<result>450</result>