ANSWER: 10


N(N-1)/2 for N=5 gives 5(5-1)/2 = 5(4)/2 = 20/2 = 10

OR

  Henry Iverson Jenkins Kent Lawson
Henry ---------- 1 2 3 4
Iverson ---------- ---------- 5 6 7
Jenkins ---------- ---------- ---------- 8 9
Kent ---------- ---------- ---------- ---------- 10
Lawson ---------- ---------- ---------- ---------- ----------

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