1. **State the problem:** A car rental agency charges 20 per day to rent a car. We need to write the cost as a function of the number of days rented and find the cost for 5 days. 2. **Define the variable:** Let $d$ represent the number of days the car is rented. 3. **Write the function:** The cost $C$ is 20 times the number of days rented, so $$C(d) = 20d$$ 4. **Calculate the cost for 5 days:** Substitute $d=5$ into the function: $$C(5) = 20 \times 5 = 100$$ 5. **Interpretation:** It costs 100 to rent the car for 5 days. **Final answer:** The cost function is $C(d) = 20d$ and the cost for 5 days is 100.