A barrel of oil is 20 kilograms, use up (4/5), how many kilograms are left?
一桶油20千克，用去(4/5)，还剩下多少千克？
<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>4</output>

<result>4</result>