A bottle of liquid medicine weighs 600 grams when it is full, and 320 grams when it is half filled. How many grams does the empty bottle of liquid medicine weigh?
一瓶药水装满时重600克，装一半时重320克，装药水的空瓶重多少克？
<gadget id="calculator">320 * 2</gadget>
<output>640</output>

<gadget id="calculator">640 - 600</gadget>
<output>40</output>

<result>40</result>