Chapter 4 - Section 4.4 - Systems of Linear Equations and Problem Solving - Exercise Set - Page 320: 32

7.5 liters of 40% solution and 7.5 liters of 10% solution

40% solution (x) and 10% solution (y) $x+y=15$ $.4x+.1y=15*.25$ $.4x+.1y=3.75$ $x+y=15$ $.4x+.1y=3.75$ $.4x+.1y=3.75$ $10*(.4x+.1y=3.75)$ $4x+y=37.5$ $x+y=15$ $-1*(x+y=15)$ $-x-y=-15$ $4x+y=37.5$ $-x-y=-15$ $-x-y+(4x+y)=-15+37.5$ $3x=22.5$ $3x/3 =22.5/3$ $x=7.5$ $x+y=15$ $7.5+y=15$ $y=7.5$