Answer: 12.5 liters of 70% hydrogen peroxide solution and 37.5 liters of 90% hydrogen peroxide solution is required.
Explanation:
Let x = Quantity of 70% hydrogen peroxide solution.
y = Quantity of 90% hydrogen peroxide solution.
To obtain: 50 liters of a 85% solution.
x+y =50 (i) (Total quantity of solution)
0.70x+0.50y=0.85(50) (Total quantity of hydrogen peroxide)
⇒ 0.70x+0.50y= 42.5 (ii)
Multiply 0.70 to (i) , we get

Subtract (iii) from (ii), we get

Put value of y in (i) , we get

Hence, 12.5 liters of 70% hydrogen peroxide solution and 37.5 liters of 90% hydrogen peroxide solution is required.