What are the odds against getting a head and a tail in a single toss of two coins?
possible outcomes:
HH, HT, TH, TT
so the prob of (head and tail) = 2/4 = 1/2
and the prob (not head and tail) = 1 - 1/2 = 1/2
so the odds against a head and a tail = (1/2) : (1/2) = 1:1