Well, simple. Let's start out with a simple substraction.
Since I know Zoe has 20 jollie ranchers, and she gave 8 to her sister, than that means 8 was taken from her.
So, I'll subtract 20 and 8.
Then, I'll subtract it. So, I'll take my 8, and count backward.
After I'm done counting backwards, I should end out with my answer, 12.