Answer:
7 minutes
Explanation:
average time = 6 minutes per mile
sum = average * count = 6 minutes / mile * 4 miles = 24 minutes
first + last mile = 5 + 5 = 10 minutes
first + second + third + last mile = sum = 24 minutes
second + third = sum - first - last = 24 - 5 - 5 = 24 - 10 = 14 minutes
average of 2 miles in the middle = sum / count = 14 minutes / 2 = 7 minutes