A bottle of oil weighs 3.4 kilograms. After half of the bottle is used, the bottle still weighs 1.9 kilograms. How many kilograms of oil are there?
一瓶油连瓶重3.4千克，用去一半后，连瓶还重1.9千克，原来有油多少千克．
<gadget id="calculator">3.4 - 1.9</gadget>
<output>1.5</output>

<gadget id="calculator">1.5 * 2</gadget>
<output>3</output>

<result>3</result>