Question 390593
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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)