Additional question: When reading a storybook, the ratio of pages read to unread is 4:5. If you read another 10 pages, you have read half of the book. How many pages does this storybook have?
附加题：读一本故事书，已读的和未读的页数的比是4：5，如果再读10页，正好读了全书的一半，这本故事书有多少页？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</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">10 / (1/18)</gadget>
<output>180</output>

<result>180</result>