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16. Which two letters come next?
  Y N U J Q F ? ?
 

Explanation:
There are two alternating sequence. The first sequence begins with a letter to W, the second sequence begins with the letter T. In both sequences, each time subtracted 4. (U = Y - 4; J = N - 4; ...; M = Q - 4; B = F - 4). Using the alphabetical order of the letters.