Xiao Ming reads a story book. He reads (1/4) of this book on the first day, and (2/5) of this book on the second day.
小明看一本故事书，第一天看了这本书的(1/4)，第二天看了这本书的(2/5)，两天后，还剩这本书的多少没有看．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (1/4) - (2/5)</gadget>
<output>7/20 = around 0.35</output>

<result>7/20 = around 0.35</result>