Final answer:
The stopping distance at 20 mph must consider both the distance covered during the driver's reaction time and the braking distance.
Step-by-step explanation:
To calculate the stopping distance at 20 mph including reaction time, we must consider both the distance the car travels during the driver's reaction time and the distance it takes to come to a complete stop once braking begins. In physics, stopping distance is calculated based on the speed of the vehicle, the deceleration rate, and the driver's reaction time. Assuming a constant velocity during the driver's reaction time, the car will travel the same distance as it does at the beginning of the stopping sequence.