A train is loaded with coal, the ratio of the loaded coal to the unloaded coal is 1:4, and then 20 tons are loaded, the ratio of the loaded coal to the unloaded coal is 1:3, how many tons of this pile of coal?
一辆火车装煤，已装的与未装的煤之比是1：4，再装20吨，已装的与未装的煤之比是1：3，这堆煤有多少吨？
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 + 4</gadget>
<output>5</output>

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

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">20 / (1/20)</gadget>
<output>400</output>

<result>400</result>