A barrel of oil weighs 12 kilograms. After half of it is used, the barrel weighs 7 kg. How many kilograms does the oil weigh?
一桶油，连桶重12千克．用去一半后，连桶重7千克．油重多少千克？
<gadget id="calculator">12 - 7</gadget>
<output>5</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>