After a barrel of gasoline is used up (2/5), there are still 30 liters left. How many liters is this barrel of gasoline originally?
一桶汽油用去(2/5)后，还剩下30升．这桶汽油原来有多少升？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">30 / (3/5)</gadget>
<output>50</output>

<result>50</result>