题目

the salty water becomes 3% concentrated after adding some water at the first time.then after
the salty water becomes 3% concentrated after adding some water at the first adding the same grams of water,it becoms 2% concentrated..what's the concentraction of the salty water after adding the same grams of water at the third time?

提问时间：2020-12-07

答案

设有溶液a克,溶液中的溶质有x克,每次加水y克.
则根据条件可以列出方程：
x/(a+y)=3%,x/(a+2y)=2%
解得y=a,x=0.06a.
所以第三次加水时,浓度为x/(a+3y)=0.06a/(a+3a)=1.5%
So the concentraction of the salty water after adding the same grams of water at the third time is 1.5%.