设平坡路为x千米,上坡路y千米,下坡路z千米.x+y+z=230 x/60+y/40+z/50=4.5 x/60+y/50+z/40=4.45 化简,得x+y+z=230 ①10x+15y+12z=2700 ②10x+12y+15z=2670 ③②-③得3y-3z=30y=10+z ④把④代入①,②并化简得x+2z=220 ...