A bottle of oil weighs 3.5 kilograms. After half of the bottle is used, the weight of the bottle is 1.9 kilograms. How many kilograms of oil are there?
一瓶油连瓶重3.5千克，用去一半后，连瓶重1.9千克，原来有油多少千克．
<gadget id="calculator">3.5 - 1.9</gadget>
<output>1.6</output>

<gadget id="calculator">1.6 * 2</gadget>
<output>3.2</output>

<result>3.2</result>