Xiao Ming read a 400-page story book, and after reading part of it, there were 180 pages left. What percentage of this book did Xiao Ming read?
小明读一本400页的故事书，读了一部分后还剩180页没有读．小明读了这本的百分之几？
<gadget id="calculator">400 - 180</gadget>
<output>220</output>

<gadget id="calculator">220 / 400</gadget>
<output>11/20 = around 0.55</output>

<result>11/20 = around 0.55</result>