I need help with combinatorics problem. The task is this: There are 9 numbers which are: 1,3,5,2,4,6,8,10,12. I need to group these numbers in 3 sets with 3 elements in every set, but there is one condition, the sum of the elements in every group needs to be odd.
I don't know what should i use, i think permutation but i don't know how to set up the answer!
Thanks very much!