Question 250779
This your classic RTD problem. Let's set up the rtd table. First notice that the units of length are not the same 45 km = 45,000 m

label . . . rate . . . time . . . distance
T1 . . . . 12 . . . . t . . . . . 12t
T2 . . . . 18 . . . . t . . . . . 18t
total . . . . . . . . 2t . . . . .45000

Now 30t = 45,000 - - > t = 15,000 sec = 250 minutes or 4hr 10 min.