Answer:
Sarah must has to ride with average speed 10 miles per hour.
Explanation:
Average Speed tells the rate at which object is travelling his whole journey.
![Average\ Speed\=\ (Total\ distance\ traveled)/(Total\ time\ taken)](https://img.qammunity.org/2020/formulas/mathematics/high-school/tlqvjp49ptv89zeaj0sq0uuww86hykjov0.png)
∴
![Average\ Speed=(2.5)/((15)/(60))](https://img.qammunity.org/2020/formulas/mathematics/high-school/ifqbtdlzt66umn75oiz2yynqfzi8m9ade2.png)
⇒
![Average\ Speed=(2.5)/(15)* 60](https://img.qammunity.org/2020/formulas/mathematics/high-school/qahcdqzryf4w95vj07uih1998oq089xfsd.png)
⇒ Average Speed = 10 miles per hour
Hence, Sarah has to travel with average speed more than 10 miles per hour.