Answer:
t1 = S1 / V1 = 2 km / 3 km/hr = 2/3 hr where S1 = 2 km
S2 = 12 km/hr * 3/2 hr = 18 km where t2 = 3/2 hr
t3 = S3 / V3 = 55 km / 30 km/hr = 11/6 hr where S3 = 55 km
T = t1 + t2 + t3 = 4/6 + 9/6 + 11 / 6 = 24/6 = 4 hrs
S = S1 + S2 + S3 = 2 + 18 + 55 = 75 km
V = S / T = 75 km / 4 hrs = 18.75 km/hr for the average speed
