Add 3 kg of water to brine to obtain a brine with a concentration of 10%, then add 1 kg of salt to the obtained brine, and the concentration of the brine is 20%, what is the original concentration of the brine?
把3千克水加盐水中，得到浓度为10%的盐水，再把1千克盐加到所得到的盐水中，这时盐水浓度为20%，原来的盐水浓度是多少．
<result>16</result>