How many grams of water can be added to 100 g of brine with a concentration of 80% to obtain a brine with a concentration of 20%.
向浓度为80%，重量为100克的盐水中加多少克水，才能得到浓度为20%的盐水．
<gadget id="calculator">400 - 100</gadget>
<output>300</output>

<result>300</result>