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Let x represent her speed uphill
Question states***  Note:  D = r*t  Distance the same uphill and downhill 
  x*6 = (x+1mph)*3
Solving for x
  3x = 3
   x = 1mph, speed uphill, downhill her speed is 2mph
CHECKING our Answer***
  1mph*6 = 2mph*3 = 6mi