Kim used 3 caramels in each party bag.
let's break this down into equations.
We know that Kim filled 3 party bags, each containing 4 chocolates and some number of caramels represented by 'c'. So, the total number of chocolates she used is 3 bags * 4 chocolates per bag = 12 chocolates.
The total number of caramels would be 3 bags * 'c' caramels per bag = 3c caramels.
And we're given that the total number of chocolates and caramels combined is 21 pieces of candy.
Therefore, the equation representing this situation is:
Total chocolates + Total caramels = Total candies
12 chocolates + 3c caramels = 21 candies
Now, we can solve for 'c':
12 + 3c = 21
3c = 21 - 12
3c = 9
c = 9 / 3
c = 3
So, Kim used 3 caramels in each party bag.