A barrel of oil weighs 30.5 kilograms, and after half of the oil is used, the weight of the barrel is 15.5 kilograms. How much does the barrel weigh?
一桶油连桶重30.5千克，用去一半油后连桶重15.5千克，桶有多重？
<gadget id="calculator">15.5 * 2</gadget>
<output>31</output>

<gadget id="calculator">31 - 30.5</gadget>
<output>0.5</output>

<result>0.5</result>