When reading a book, I read 30% of the whole book on the first day, and 1/4 of the whole book on the second day, leaving 90 pages unread. How many pages does this book have?
看一本书，第一天看了全书的30%，第二天看了全书的(1/4)，还剩下90页未看．这本书共有多少页？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

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

<gadget id="calculator">1 - (3/10) - (1/4)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">90 / (9/20)</gadget>
<output>200</output>

<result>200</result>