Question

There is a beaker of 3.5% acid solution and a beaker of 6% acid solution in the science lab. Mr. Larson needs 200 milliliters of a 4.5% acid solution for an experiment. How many milliliters of each solution should he combine?

Answer 1

Et x = the volume of the 3.5% solution 
Let y = the volume 6% solution 

(3.5%)x + (6%)y = (4.5%)(200) 
0.035)x + 0.06y = 0.045(200) 
0.035x + 0.06y = 9 ( Equation 1 )

x + y = 200 ( Equation 2 )
x = 200 - y 

Substitute x = 200 - y into equation 1: 
0.035x + 0.06y = 9 
0.035(200 - y) + 0.06y = 9 
7 - 0.035y + 0.06y = 9 
0.06y - 0.035y = 9 - 7 
0.025y = 2 
y = 2/(0.025) 
y = 80 mL (the volume of 6% solution.)

Substitute y = 80 into equation 2: 
x + y = 200 
x + 80 = 200 
x = 200 - 80 
x = 120 mL (the volume of the 3.5% solution.)

Therefore, Mr. Larson should combine 120 mL of the 3.5% solution 
and 80 mL of the 6% solution to make 200 mL of 4.5% solution. 

Answer is 120 mL of the 3.5% solution & 80 mL of the 6% solution

Hope I helped :)