Let [tex]x[/tex] be the number of liters of the 10% solution to be used, and [tex]y[/tex] the number of liters of the 4% solution. The chemist wants to end up with a 4 liter solution, so
[tex]x+y=4[/tex]
For each liter used in the mixture, a concentration of either 10% or 4% acid will be contributed, and the goal is to make a 4L solution whose concentration is 7%, which means
[tex]0.1x+0.04y=0.07(x+y)=0.28[/tex]
Solving for [tex]x,y[/tex] gives [tex]x=y=2[/tex], so 2L of both solutions is needed.
