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Hi,  
   Walter is 3 times as old as Marti   
Let x and 3x represent Marti's and Walter's ages NOW respectivey
Write as we Read*** ( ten years from now)
  3x + 10 = 2(x+10)
solving for x
 3x + 10 = 2x + 20
      x = 10, Marti's age NOW.  Walter is NOW 30.  (3*10)
CHECKING Our Answer****
  40 = 2(10+10) = 40