A barrel of oil weighs 9 kilograms. After half of the oil is poured out, the weight of the barrel is 5.5 kilograms. How many kilograms does the barrel weigh?
一桶油连桶重9千克，倒出一半油以后，连桶重5.5千克，桶重多少千克？
<gadget id="calculator">9 - 5.5</gadget>
<output>3.5</output>

<gadget id="calculator">5.5 - 3.5</gadget>
<output>2</output>

<result>2</result>