There are 12 empty juice bottles. You drank 8 bottles of juice. I drank the rest. What fraction did i drink?

If the other person drank 8 bottles, that means that there were 4 bottles left

(12 - 8 = 4 bottles)

So you drank 4 out of 12, or 4/12 of the bottles, which simplifies to 1/3 of the bottles.