Hi,  

 he drove 397 miles      

Let x and (397mi-x) represent the highway miles and city miles respectively

Question states***

  x/36mpg + (397mi-x)/31mpg = 12gal

 31x + 36(397-x) = 12*36*31

   -6x + 36*397 = 12*36*31

         36*397-12*36*32 = 6x

           468 = 6x

             x = 78mi, the highway miles.  City miles were 319mi.  (397-78)

               



 

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