A barrel of oil uses 4 kilograms, which is exactly (2/5) of this barrel of oil. How many kilograms of this barrel of oil are left?
一桶油，用了4千克，正好是这桶油的(2/5)，这桶油还剩多少千克？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">4 / (2/5)</gadget>
<output>10</output>

<gadget id="calculator">10 - 4</gadget>
<output>6</output>

<result>6</result>