A barrel of oil weighs 8.5 kilograms in total. After half of the barrel is used up, there is 4.9 kilograms left in the barrel. How much does the barrel weigh?
一桶油，连桶共重8.5千克，用去一半后连桶还剩4.9千克，桶重多少千克？
<gadget id="calculator">8.5 - 4.9</gadget>
<output>3.6</output>

<gadget id="calculator">4.9 - 3.6</gadget>
<output>1.3</output>

<result>1.3</result>