A barrel of gasoline was used (1/5) on the first day, and the rest (1/2) was used on the second day, leaving 400 liters useless. How many liters are there in this barrel of gasoline?
一桶汽油，第一天用了(1/5)，第二天用了剩下的(1/2)，还剩400升没有用，这桶汽油共有多少升？
<gadget id="calculator">1 / 5</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">1 / 2</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">(4/5) * (1/2)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">400 / (2/5)</gadget>
<output>1_000</output>

<result>1_000</result>