这是一道关于浓度问题的数学题：浓度为百分之8的盐水40kg，要配置含盐百分之20的盐水100kg，需加入的盐水浓度为百分之几？刚学，不明白，请老师帮忙解答一下。

数学题解答：

设问题中的浓度为x。（注：它是一个百分数）

等量关系式：原来盐水中含的盐+加入的盐水中含的盐＝配成的盐水中含的盐

方程：40×8%　+　（100-40）×x　＝　100×20%

解这个方程：x=28%

答：需加入的盐水浓度为28%。

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