Xiaolan reads a book. She reads 20% of the book on the first day and 12 pages on the second day. At this time, the ratio of the number of pages she has read to the total number of pages in the book is 1:3. how many pages
小兰读一本书，第一天读了全书的20%，第二天读了12页，这时已读的页数与全书的总页数的比是1：3，这本书一共有多少页？
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/3) - (1/5)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">12 / (2/15)</gadget>
<output>90</output>

<result>90</result>