Distance along northern path      Dn  =   1.2 D
 
   Distance along southern path      Ds  =   1.6 D

   Speed of cars                      v  =   60 mph



                                               Dn
       Time for northern car to drive  Tn  =  -----
                                               v

                                               Ds
       Time for southern car to drive  Ts  =  -----
                                               v

                                                            Ds - Dn
       Difference in time       (delta T)  =   Ts - Tn  =  --------- 
                                                               v

                                                 1.6 D - 1.2 D       0.4 D
                                           =    ---------------  =  -------
                                                       v               v


    But we know that difference: it's 0.5 hours. So we can solve
    for the unknown distance D.

                         v                       60 mph
                 D  =  ------ * (delta T)   =   -------- * 0.5 hour
                        0.4                        0.4


                            miles
                 D  =   150 -----  * (0.5 hour)  =   75 miles
                             hour