Pour the milk into the same empty bottle. If 4 cups of milk are poured in, the weight of the bottle is 500 grams. If 6 cups of milk are poured in, the weight of the bottle is 670 grams. Think about it: how many grams does this empty bottle weigh?
将牛奶倒入同一个空瓶里．如果倒入4杯牛奶，则连瓶重500克，如果倒入6杯牛奶，则连瓶重670克，想一想：这个空瓶重多少克．
<gadget id="calculator">670 - 500</gadget>
<output>170</output>

<gadget id="calculator">6 - 4</gadget>
<output>2</output>

<gadget id="calculator">170 / 2</gadget>
<output>85</output>

<gadget id="calculator">85 * 4</gadget>
<output>340</output>

<gadget id="calculator">500 - 340</gadget>
<output>160</output>

<result>160</result>