Answer: Her average speed would be 8.89 miles per hour.
Explanation:
Since we have given that
Time taken to run from mile marker 1 to mile marker 2 = 6 minutes
Time taken to run from mile marker 2 to mile marker 4 = 21 minutes
So, Number of miles from 1 to 4 = 4 miles
total time taken = 6+21 = 27 minutes
So, Average speed would be

Hence, her average speed would be 8.89 miles per hour.