For a pile of goods, 20% of the total amount was transported for the first time, and 30% for the second time, leaving 400 tons undelivered. How many tons are there in total for this pile of goods?
一堆货物，第一次运走了总数的20%，第二次运走的30%，还剩下400吨没有运，这堆货物共有多少吨？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/5) + (3/10)</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">400 / (1/2)</gadget>
<output>800</output>

<result>800</result>