Answer:
7 seconds
Step-by-step explanation:
In 1 second, the distance covered by the car is 3 feet.
We know that:
![\text{Average Speed=}\frac{\text{Distance}}{\text{Time}}](https://img.qammunity.org/2023/formulas/mathematics/college/31xs9t6kwe36giakpqmajr7grb6l2e0riq.png)
Therefore, the average speed of the car will be:
![S\text{peed}=(3)/(1)=3\text{ ft per second}](https://img.qammunity.org/2023/formulas/mathematics/college/ayefv4asfh7g95vyt504wskj4ilosiezrj.png)
We want to determine the time when:
Distance = 21 feet
Substitution into the formula for average speed gives us:
![\begin{gathered} 3\text{ ft per second = }\frac{21}{\text{Time}} \\ 3*\text{Time}=21 \\ \text{Divide both sides by 3} \\ \text{Time = 7 seconds} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/4p3rwcgf3oo3wq2jmr3txvffql0iomjrd9.png)
Therefore, it takes 7 seconds to drive 21 feet.