Xiaohui reads a book. The ratio of the number of pages she has read to the number of pages she has not read is 3:5. She has read 21 pages. How many pages are there in total?
小惠读一本书，已读的页数和未读的页数比是3：5，她已经读完了21页，这本书一共有多少页？
<gadget id="calculator">3 / 8</gadget>
<output>3/8 = around 0.375</output>

<gadget id="calculator">21 / (3/8)</gadget>
<output>56</output>

<result>56</result>