What are the odds against throwing either a sum of 3 or 8 in rolling a pair of regular dice?
you can get the sum by
2,1
1,2
3,5
5,3
6,2
2,6
4,4
each of these has a probility of 1/36, so the total probability is 1/36 * 7 or 7/36
So of 36 tries, you get the sum 7 times and don't get it 29
Odds? 7:29
odds against? 29:7