A pile of coal, after using (4/5), there are still 31 tons left. How many tons is there in this pile of coal?
一堆煤，用了(4/5)后还剩31吨，这堆煤原来有多少吨？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">31 / (1/5)</gadget>
<output>155</output>

<result>155</result>