1. **State the problem:** Marcy burns 80% of the calories walking compared to jogging. We want to write a function for calories burned walking as a function of calories burned jogging, and find how many calories she burns walking if she burns 120 calories jogging. 2. **Write the function:** Let $J$ be the calories burned jogging. Let $W$ be the calories burned walking. Since walking burns 80% of jogging calories, the function is: $$W = 0.8J$$ 3. **Calculate the calories burned walking when jogging burns 120 calories:** Substitute $J = 120$: $$W = 0.8 \times 120$$ $$W = 96$$ 4. **Interpretation:** Marcy burns 96 calories walking if she burns 120 calories jogging. **Final answer:** $$W = 0.8J$$ $$W = 96 \text{ calories when } J = 120$$