Put 20 grams of salt into 100 grams of water. After standing for four days, the weight of the salt water is only 100 grams. What is the concentration of the salt water at this time?
20克盐放入100克水中，放置四天后，盐水重量只有100克，这时盐水的浓度是多少？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">100 / 100</gadget>
<output>1</output>

<gadget id="calculator">(1/5) * 1</gadget>
<output>1/5 = around 0.2</output>

<result>1/5 = around 0.2</result>