Xiao Ming reads a story book, the ratio of the number of pages he has read to the number of unread pages is 3:5, he has read 21 pages, how many pages are left?
小明读一本故事书，已读页数和未读页数的比是3：5，他已经读完21页，还有多少页没有读？
<gadget id="calculator">5 / 3</gadget>
<output>5/3 = around 1.666667</output>

<gadget id="calculator">21 * (5/3)</gadget>
<output>35</output>

<result>35</result>