Answer:
1.14 liters
Explanation:
We have 8% glycerin solution and we need to make it 16% glycerin solution [more concentration].
When we add "x" liters, we add glycerin as well as changes the total. in 12 Liters, there is 12 * 0.08 = 0.96Liters of glycerin. How much we need to add to this??
Note, 16% = 0.16
We can write a Ratio as shown below, cross multiply and solve for x:

We need to add around 1.14 liters