We know that the formula for calculating distance is:
distance d = velocity v * time t
Calculating for each interval:
1: 25 km/hr for 4 minutes
d1 = (25 km / 60 min) (4 min) = 1.67 km
2: 50 km/hr for 8 minutes
d2 = (50 km / 60 min) (8 min) = 6.67 km
3: 20 km/hr for 2 minutes
d2 = (20 km / 60 min) (2 min) = 0.67 km
The total distance covered is:
d = d1 + d2 + d3
d = 1.67 + 6.67 + 0.67
d = 9 km
The average speed is:
s = 9 km (1000 m / km) / (14 min * 60 s / min)
s = 10.72 m / s