8 miles = 12874.8m
Total distance = 16 miles = 25749.5m
time = distance/velocity
First instance, t =12874.8/24
= 536.45
Second instance, t = 12874.8/30
= 429.16
Total time = 536.45 + 429.16 = 965.61
Average velocity = Total distance / Total time
= 25749.5/965.61
= 26.67 m/s