Let [tex]x[/tex] be the number of liters of the 10% solution she needs to use. She'd end up with a 6% solution with a volume of [tex]9+x[/tex] liters. The starting solution contains 0.04*9 = 0.36 liter of salt. Each liter of the 10% solution contributes 0.1 liter of salt, so that
[tex]0.36+0.1x=0.06(9+x)[/tex]
Solve for [tex]x[/tex]:
[tex]0.36+0.1x=0.54+0.06x[/tex]
[tex]0.04x=0.18[/tex]
[tex]x=4.5[/tex]
so Mayra needs to add 4.5 liters of the 10% solution.
