Use 1 cup to pour water into 1 empty bottle. If 3 cups of water are poured into it, the total weight of the bottle is 440 grams; if 7 cups of water are poured into it, the total weight of the bottle is 600 grams. How much does the empty bottle weigh in grams?
用1个杯子向1个空瓶里倒水，如果倒进3杯水，连瓶共重440克；如果倒进7杯水，连瓶共重600克．问空瓶重多少克．
<gadget id="calculator">600 - 440</gadget>
<output>160</output>

<gadget id="calculator">160 / 4</gadget>
<output>40</output>

<gadget id="calculator">3 * 40</gadget>
<output>120</output>

<gadget id="calculator">440 - 120</gadget>
<output>320</output>

<result>320</result>