X + y = 20 --> eqn 1 The second equation would be the mass balance shown below: (x)(0.40) + y(0.70) = 20(0.52) --> eqn 2 Rearranging eqn 1,x = 20 - ySubstitute this to eqn 2: (20 - y)(0.40) + y(0.70) = 20(0.52)Solve for y,y = 8 grams Finally, solve for x.x = 20 - 8 = 12 grams Therefore, Serena needs 12 grams of the 40% solution and 8 grams of the 70% solution.