Let the armored vehicle speed be x Km/h, car A lead by y minutes, and x(y+40)=90*40, that is, xy+40x=3600 x(y+90)=80*90, that is, xy+90x=7200. If the xy term is cancelled, the solution of 3600-40x=7200-90x can be obtained, that is, 50x=3600 and x = 72km/.