The area of a piece of colored paper is 45 square decimeters, and 20 square decimeters have been used. How much of this colored paper is left is useless.
一张彩纸的面积是45平方分米，已经用去20平方分米．还剩下这张彩纸的多少没有用．
<gadget id="calculator">45 - 20</gadget>
<output>25</output>

<gadget id="calculator">25 / 45</gadget>
<output>5/9 = around 0.555556</output>

<result>5/9 = around 0.555556</result>