Chapter 4 - Section 4.4 - Systems of Linear Equations and Problem Solving - Exercise Set - Page 322: 54

c, d

If we don't mix any of the 50% solution with the 100% solution, we would have a 100% solution. If we don't mix any of the 100% solution with the 50% solution, we would have a 50% solution. Since any combination of the two solutions would result in a solution between 50% and 100%, the only options possible are 62% and 90%.