barrel of oil weighs 16 kilograms. After half of it is used, the barrel weighs 9 kilograms. How many kilograms does the barrel weigh?
一桶油连桶重16千克，用去一半后，连桶重9千克，桶重多少千克？
<gadget id="calculator">16 - 9</gadget>
<output>7</output>

<gadget id="calculator">9 - 7</gadget>
<output>2</output>

<result>2</result>