There is a pile of coal, after 20% of the total amount is used, another 300 tons are brought in. At this time, the mass ratio of the coal to the original mass is 13:10. How many tons is this pile of coal originally?
有一堆煤，用去总数的20%后又运来300吨，此时煤的质量与原来的质量比是13：10．这堆煤原来有多少吨．
<gadget id="calculator">13 / 10</gadget>
<output>13/10 = around 1.3</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(13/10) - (4/5)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">300 / (1/2)</gadget>
<output>600</output>

<result>600</result>