To place 3 non-consecutive positions: let the positions be $ x_1 < x_2 < x_3 $, with $ x_{i+1} \ge x_i + 2 $. Let $ y_i = x_i - (i-1) $, then $ y_1 < y_2 < y_3 $ in $ \{1,\dots,6\} $. So number is $ \binom{6}{3} = 20 $.

Thu, 30 Apr 2026 13:59:36 +0000

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