A pile of coal, 30% is used on the first day, and the rest (3/7) is used on the second day, leaving 4 tons. How many tons is this pile of coal originally?
一堆煤，第一天用去30%，第二天用去剩下的(3/7)，还剩下4吨，这堆煤原来有多少吨？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(7/10) * (3/7)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10) - (3/10)</gadget>
<output>2/5 = around 0.4</output>

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

<result>10</result>