If (3/5) of a pile of coal is used up and there are still 4 tons left, then how many tons does this pile of coal have?
一堆煤用去了它的(3/5)后还剩下4吨，那么这堆煤原有多少吨．
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">4 / (2/5)</gadget>
<output>10</output>

<result>10</result>