a) T=t1-t3=9h 50 min-8h 30 min=1h
20min =4/3h
T'=d/v1=18/54=1/3h
tr=T-t'-t2=4/3-1/3-1/4=16/12-4/12-3/12=9/12=3/4h
dr=D-d1=85,5-18=67,5km
v2=dr/tr=67,5/3/4=67,5*4/3=22,5*4=90km/h
b)vm=d1+dr/t'+t1=18+67,5/1/3+3/4=85,5/4/12+9/12=85,5/13/12=85,5*12/13=1026/13=~78,92(aproximativ)