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13. Which letter should replace the question mark?
  IQ test. Fast and free online iq test # 1 with answers and solutions. Question # 13. Which letter should replace the question mark?
 

Explanation:
Opposite letters differ by a factor of 2 and - 1. In terms of the alphabetic order of letters. (U = K × 2 - 1; E = C × 2 - 1; S = J × 2 - 1; I = E × 2 - 1).