MasterMind Puzzle:

Solve the hidden code.

(Note: Regular MasterMind conditions apply:

• 6 colors (green, red, yellow, brown, dark-blue, light-blue)
• A black keypeg indicates that a code peg is the right color and in the right position
• A white keypeg indicates one of the code pegs is the right color but in the wrong position

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  Answer:
  
  
  In the second row there was one color change from the first row; red was taken out and replaced with yellow... and this guess received one more keypeg. This means yellow is in and red is out. And if yellow is in, obviously it must be in column 3.
  
  
  There was one color change from the second and fourth rows... dark-blue was taken out and replaced with brown. Since the number of keypegs stayed the same, this means they are either both in or both out. So if they are both in then green and light-blue must both be out, according to the information received with the fourth guess. (Since it must be yellow and brown that are drawing the two keypegs.) But how can green and light-blue both be out if two black keypegs were received in the third guess? They can't so this means that dark-blue and brown must both be out, not in.
  
  
  So, red is out, dark-blue is out, brown is out
  Yellow is in.
  One of green or light-blue is in but not both.
  A double of green is out (Row 3)
  
  
  So the four colors must be either:
  
  
  yellow, light-blue, light-blue, light-blue
  yellow, yellow, yellow, green
  yellow, yellow, light-blue, light-blue
  yellow, yellow, yellow, light-blue
  
  A triple with light-blue just doesn't work.
  
  
  If green is in it can't be in columns 3 or 4. A simple check indicates no solution fits with it in either columns 1 or 2 either so this color combination is not correct.
  
  
  If light-blue is in twice one must be in column 4 and the other must be in column 2 since light-blue can't be in column 1 (row 1) or column 3. But yellow, light-blue, yellow, light-blue would not receive a black keypeg in row two, which it did.
  
  
  The only combination left happens to work. yellow, yellow, yellow, light-blue fits each guess.