Let us assume that x kg of 30% salt solution is mixed with (200-x) kg of 40% salt solution in order to obtain 200 kg of 37% salt solution.
We can set up a linear equation in terms of x by setting amount of total salt before mixing and after mixing equal to each other.
[tex]30\%\text{ of }x+40\%\text{ of }(200-x)=37\%\text{ of }200\\0.30x+0.40(200-x)=0.37(200)\\0.30x+80-0.40x=74\\0.10x=6\\x=60[/tex]
Therefore, 60 kg of 30% salt solution should be mixed with 140 kg of 40% solution.
