t=s/v=5km / 5km/h=1h
t=s/v=4km / 4km/h=1h
t=s/v=3km / 2km/h=1.5h
v=s/t=(5+4+3+5+4+3) / (1+1+1.5+1+1+1.5)=3.429km/h