A barrel of oil weighs 5.1 kilograms, and after half of it is used, it still weighs 3.1 kilograms. What is the weight of the barrel?
一桶油连桶重5.1千克，用去一半后还重3.1千克，则桶重多少千克．
<gadget id="calculator">5.1 - 3.1</gadget>
<output>2</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">5.1 - 4</gadget>
<output>1.1</output>

<result>1.1</result>