A rope is 90 meters long, 50 meters have been used up, how much of it is useless?
一根绳子长90米，已经用去了50米，还剩下几分之几没有用？
<gadget id="calculator">90 - 50</gadget>
<output>40</output>

<gadget id="calculator">40 / 90</gadget>
<output>4/9 = around 0.444444</output>

<result>4/9 = around 0.444444</result>