How many nine-digit numbers can be made using each of the digits 1 through 9 exactly once with the digits alternating between odd and even?
If it starts with an even
There are 4 first digit possibilities (not 5 becasue it can's start with 0) , 4 third digits 3 fifth digits etc so that is 4*4! * (now for the odd digits) 4!
If it starts with an odd then there will be 5!*4! different ones
Now add them together.
4*4!*4! + 5!*4!