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23. Which letter should replace the question mark?
  IQ test. Free iq test online # 2 with answers and solutions. Question # 23. Which letter should replace the question mark?
 

Explanation:
C = B × B - 1; G = C × C - 2; M = D × D - 3; U = E × E - 4. First letters: B, C, D, E. Opposite letter = (First letter multiplied by itself) - 1 (further 2, 3, 4). In terms of the alphabetic order of letters.