A glass of water weighs 900 grams, and after half of the water is poured out, it still weighs 500 grams. How many grams does the water glass weigh?
一杯水，连杯重900克，倒出水的一半后还重500克，则水杯重多少克？
<gadget id="calculator">900 - 500</gadget>
<output>400</output>

<gadget id="calculator">400 * 2</gadget>
<output>800</output>

<gadget id="calculator">900 - 800</gadget>
<output>100</output>

<result>100</result>