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# help

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How many arrangements of the numbers $$1, 2, 3, \dots, 7$$ are there where the sum of any two adjacent numbers is odd?

Mar 22, 2019

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$$\text{the first number must be odd}\\ 4 \cdot 3 \cdot 3 \cdot 2 \cdot 2 = 144\\$$

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Mar 22, 2019

$$\text{the first number must be odd}\\ 4 \cdot 3 \cdot 3 \cdot 2 \cdot 2 = 144\\$$