Question

How many liters of peroxide should be added to 12 liters of an 8% peroxide solution so that the remaining solution contains 16% peroxide?

Answer 1

The question above may be solved using the overall mass and component balances. If we let x be the  number of liters of peroxide that should be added, the final mixture will have a quantity of x+12 L. The peroxide balance before and after mixing is,
                            x(1) + 12(0.08) = (x + 12)(0.16)
The value of x is 1.143. Thus, approximately 1.143 L of peroxide should be added to the original solution. 

Answer 2

Lets say you add x liters of peroxide
then the total number of liters will be 12+x
presently you have 8% of 12 or .96 liters of peroxide
when you add x liters you will have .96+x liters of peroxide total
you want that to be 16% of (12+x) so set

.96+x=.16(12+x)
solve for x
multiply both sides by 100

96 + 100x = 16(12+x)
96 + 100x = 192 + 16x
84x = 96
x = 1.19

So 1.19 liters