Question 200265
Since the 2 solutions must add to 1 liter, then they must add to 1,000 ml

Let the amount of 1% salt solution in the 2.8% salt solution be x. 
Then the amount of 4% solution in the 2.8% salt solution is 1,000 – x


Therefore, we get:  .01(x) + .04(1,000 – x) = .028(1,000)


This equation becomes:  .01x + 40 - .04x = 28

-.03x  +  40  =  28
-.03x   =   - 12
{{{x  =  (-12)/(-.03)}}} = 400

Therefore, he needs to mix 400 ML of the 1% salt solution, and 600 (1,000 – 400) ML of the 4% salt solution to get 1,000 ML, or 1 Liter of 2.8% salt solution.