A barrel of oil weighs 25kg, how many kilograms are left when (4/5) of the barrel of oil is used up?
一桶油重25kg，用去了这桶油的(4/5)，还剩多少千克？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">25 * (4/5)</gadget>
<output>20</output>

<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<result>5</result>