The equation for speed is
![v\text{ = }(x)/(t)](https://img.qammunity.org/2023/formulas/mathematics/college/xtm6kp54y5130annxa89mgbu8wezivzvui.png)
where x is the distance, and t is the time. In the international system, we have that x must be in mt, and t must be in seconds. But in this case, we don't use the international system. So:
![v\text{ = }\frac{99\text{ miles}}{\frac{11\text{ hours}}{2\text{ }}}\text{ = }\frac{99\text{ miles x 2}}{11\text{ hours}}\text{ = 18 miles/hour}](https://img.qammunity.org/2023/formulas/mathematics/college/cz3cfa6iyj4in3n4p8rnknwofp94hpclmx.png)
We can conclude that the correct answer is 18 miles/ hour.