15m/s=54km/h
s1=54kmm/h*1/3h=18km
s2=72m/h*0.5h=36km
s=s1+s2=54km
t=1/3h+1/2h=5/6h
v=s/t=64.8km/hs1=54kmm/h*1/3h=18kms2=72m/h*0.5h=36kms=s1+s2=54kmt=1/3h+1/2h=5/6h过程怎么算的？15m/s=54km/h20min=1/3h第一段路程s1=54kmm/h*1/3h=18km第二段路程s2=72km/h*0.5h=36km总路程s=s1+s2=54km总时间t=1/3h+1/2h=5/6h全程平均速度v=s/t=64.8km/h