What is probability a horse will win a race if it has odds of 5 to 1?

5 to 1 to win means the probability of actually winning is 1 to 5, or 0.2. The larger the payout, the longer (or poorer) the odds.

Actually, 5 to 1 means 1 chance in 6 or 16.7%. Of course, race track odds may not be the true probablity of winning.