Final answer:
Olga had the fastest average speed among the three runners.
Step-by-step explanation:
To find the fastest average speed, we need to divide the distance each runner ran by the time it took them. Trisha ran 10 km in 2.5 hours, so her average speed is 10 km / 2.5 h = 4 km/h.
Jason ran 7.5 km in 2 hours, so his average speed is 7.5 km / 2 h = 3.75 km/h. Olga ran 9.5 km in 2.25 hours, so her average speed is 9.5 km / 2.25 h = 4.22 km/h.
Therefore, Olga had the fastest average speed among the three runners.