Add 3 kg of water to the brine to obtain a brine with a concentration of 10%, and then add 1 kg of salt to the resulting brine to obtain a concentration of 20%. What is the original concentration of the brine?
把3千克水加到盐水中，得到浓度为10%的盐水，再把1千克盐加到所得的盐水中，这时浓度为20%，原来盐水浓度是多少？
<gadget id="calculator">0.8 / 5</gadget>
<output>0.16</output>

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

<gadget id="calculator">0.16 * 1</gadget>
<output>0.16</output>

<result>0.16</result>