A barrel of oil weighs 8 kilograms, and after half of the oil is poured out, the weight of the barrel is 4.5 kilograms. How much does this barrel of oil weigh in kilograms?
一桶油，连桶重8千克，倒出一半油后，连桶重4.5千克．这一桶油重多少千克．
<gadget id="calculator">8 - 4.5</gadget>
<output>3.5</output>

<gadget id="calculator">3.5 * 2</gadget>
<output>7</output>

<result>7</result>