There is a pile of coal, (1/6) of it is used on the first day, and the rest (6/7) is used on the second day. At this time, there are still 20 tons left. Find out how many tons this pile of coal is?
有一堆煤，第一天用去它的(1/6)，第二天用去剩下的(6/7)，这时还剩下20吨，求原有这堆煤多少吨？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 - (1/6)</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">6 / 7</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">(5/6) * (6/7)</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(5/6) - (5/7)</gadget>
<output>5/42 = around 0.119048</output>

<gadget id="calculator">20 / (5/42)</gadget>
<output>168</output>

<result>168</result>