(4/5) tons of coal are used up (13/20), what fraction of this pile of coal is left? How many.
(4/5)吨煤，用去(13/20)，还剩下这堆煤的几分之几？多少．
<gadget id="calculator">13 / 20</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">1 - (13/20)</gadget>
<output>7/20 = around 0.35</output>

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