Let [tex]x[/tex] be the volume of 20% solution, [tex]y[/tex] the volume of the 60% solution. We want a total volume of 400 mL in the final mixture, so
[tex]x+y=400[/tex]
Each mL of either solution will contribute a corresponding concentration, and in the final mixture we want the 400 mL to have a 40% concentration, which means we should also have
[tex]0.2x+0.6y=0.4\times400=160[/tex]
Solve the system and we get [tex]x=y=200\,\rm{mL}[/tex].
