Average speed is given as the ratio of total distance traveled in a particular time interval.
t = time of travel for allex = 30 min = 30/60 hour = 0.5 hour (Since 1 hour = 60 min)
d = distance traveled in time "t" = 4 km
V = average speed of allex
Average speed is given as
V = d/t
V = 4/0.5
V = 8 km/h