A barrel of oil weighs 6 kilograms, and after half of the oil is used, the barrel still weighs 3.5 kilograms. How many kilograms does the barrel weigh?
一桶油连桶重6千克，用去一半油后，连桶还重3.5千克．桶重多少千克？
<gadget id="calculator">6 - 3.5</gadget>
<output>2.5</output>

<gadget id="calculator">2.5 * 2</gadget>
<output>5</output>

<gadget id="calculator">6 - 5</gadget>
<output>1</output>

<result>1</result>