A standard oil barrel weighs 7 kg including oil. The driver, Uncle Ma, has already used up half of the oil, and now even the barrel still weighs 4 kilograms. How much oil is in the barrel.
一个标准油桶，桶连油共重7千克．司机马叔叔已经用去一半油，现在连桶还重4千克．桶里还有多少油．
<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<result>3</result>