A barrel of oil weighs 9 kg. After half of it is used, the weight of the barrel is 5 kg. What is the original weight of the oil in the barrel?
一桶油连桶重9千克，用去一半后，连桶重5千克，原来桶里的油重多少千克．
<gadget id="calculator">9 - 5</gadget>
<output>4</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<result>8</result>