A barrel of oil weighs 8 kilograms, and when half of it is used up, the weight of the barrel is 4.5 kilograms. How much does the barrel weigh?
一桶油连桶重8千克，用去一半后连桶重4.5千克，桶重多少？
<gadget id="calculator">8 - 4.5</gadget>
<output>3.5</output>

<gadget id="calculator">4.5 - 3.5</gadget>
<output>1</output>

<result>1</result>