Explanation:
If x is the volume of the 10% sulfuric acid, and y is the volume of 25% sulfuric acid, then:
x + y = 500
0.10 x + 0.25 y = 0.20 (500)
Solve the system of equations with substitution or elimination. Using substitution:
y = 500 − x
0.10 x + 0.25 (500 − x) = 0.20 (500)
0.10 x + 125 − 0.25 x = 100
25 = 0.15 x
x = 167 mL
y = 333 mL
You need 167 mL of 10% sulfuric acid and 333 mL of 25% sulfuric acid.