After a barrel of oil is used up (4/5), there is still 20 kg of gasoline in the barrel. How many kilograms have been used?
一桶油用去(4/5)后，桶里还有汽油20千克．已经用去了多少千克？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">20 / (1/5)</gadget>
<output>100</output>

<gadget id="calculator">100 * (4/5)</gadget>
<output>80</output>

<result>80</result>