barrel of oil weighs 9 kilograms, and when half of it is used, the weight of the barrel is 4.9 kilograms. How much does the empty barrel weigh in kilograms.
一桶油连桶重9千克，用去一半后连桶重4.9千克．空桶重多少千克．
<gadget id="calculator">9 - 4.9</gadget>
<output>4.1</output>

<gadget id="calculator">4.1 * 2</gadget>
<output>8.2</output>

<gadget id="calculator">9 - 8.2</gadget>
<output>0.8</output>

<result>0.8</result>