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Arrangements of 1 to 7 with evens never touching

Question

Count the orderings of the numbers 1,2,3,4,5,6,71,2,3,4,5,6,7 in which no two even numbers sit next to each other.

Short answer

14401440. Place the four odd numbers first (4!4! ways); they create 55 gaps, and the three evens must go in three different gaps: (53)=10\binom53=10 choices of gaps and 3!3! orders. Total 24⋅10⋅6=144024\cdot10\cdot6=1440.

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