How many arrangements of the numbers 1, 2, 3, ..., 7, 8, 9 are there where the sum of any two adjacent numbers is odd?
+113 It seems like swapping the even numbers around with eachother would do the trick. In this series there are 4 even numbers, they can be rearranged in 4! ways or 4x3x2x1 = 24.
