A bottle of oil weighs 800 grams, and after half of the oil is used, the weight of the bottle is 500 grams. How many grams does the bottle weigh.
一瓶油连瓶重800克，用去一半的油后连瓶重500克．瓶重多少克．
<gadget id="calculator">800 - 500</gadget>
<output>300</output>

<gadget id="calculator">300 * 2</gadget>
<output>600</output>

<gadget id="calculator">800 - 600</gadget>
<output>200</output>

<result>200</result>