barrel of oil weighs 90 kg. After half of the oil is used, the weight of the barrel is 50 kg. How many kilograms of oil are in the barrel?
一桶油连桶重90千克，用去一半油后，连桶称还重50千克．原来桶里装有多少千克的油．
<gadget id="calculator">90 - 50</gadget>
<output>40</output>

<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<result>80</result>