Answer:
They hike 9.11 km on day three
Step-by-step explanation:
Total distance traveled on a 3 day hiking trip in the mountains = 18.5 km
The students hike 0.28 of the total distance the first day.
So, Remaining distance = 18.5 - 0.28 = 18.22
Now we are given that they split the remaining distance equally between the second and third days
So, Distance traveled by them on third = Distance traveled by them on second day = Half of the remaining distance = 
So, they hike 9.11 km on day three