ANSWER: 15


N(N-1)/2 for N=6 gives 6(6-1)/2 = 6(5)/2 = 30/2 = 15

OR

  May Nash Overton Pickens Queen Rogers
May ---------- 1 2 3 4 5
Nash ---------- ---------- 6 7 8 9
Overton ---------- ---------- ---------- 10 11 12
Pickens ---------- ---------- ---------- ---------- 13 14
Queen ---------- ---------- ---------- ---------- ---------- 15
Rogers ---------- ---------- ---------- ---------- ---------- ----------

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