A certain amount of water is added to 200 grams of 10% brine to make the concentration 4%, what is the weight of the added water in grams?
向浓度为10%的盐水200克中加入一定量的水，浓度变为4%，求加入的水重量为多少克．
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4 / 100</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/10) / (1/25)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">200 * (5/2)</gadget>
<output>500</output>

<gadget id="calculator">500 - 200</gadget>
<output>300</output>

<result>300</result>