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Probability in mathematics is just a number between zero and one. For example, we say, "The probability of this happening is fifty percent."

However, in many statements, we use "chances" and "odds", which are plural as opposed to the singular "probability". For example, consider "What are the chances of that!" and "The odds of winning the lottery are minuscule."

Dictionary definitions just tell it how it is, but not why:

You refer to how likely something is to happen as the odds that it will happen.

The following structure reads especially clunky partly because it has are next to one.

The overall odds of winning a lottery prize are 1 in 13.
CED (emphasis mine)

Again, that last part is just one number (~0.08), so we would say, "The overall probability of winning the lottery is point zero eight." Yet, we use the plural "chances" and "odds" though they refer to one number. "What is the chance of that!" and "The odd of winning the lottery is minuscule" come more intuitively to me.

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    Does this answer your question? "Chances of (doing) something" vs. "chances at (doing) something"? – FumbleFingers Jan 16 at 19:02
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    Aside: for odds, we don't usually use a number beween 0 and 1, but something like "a million to one!" The odds are what a bookmaker quotes for the payout from a race. It is a plural word – its singular odd has a different meaning. – Weather Vane Jan 16 at 19:04
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    @FumbleFingersReinstateMonica I don't think that's quite the same question though. They are related, for sure, but this question is about the set phrase "chances/odds are". – Eddie Kal Jan 16 at 19:04
  • There's rarely if ever any difference between What is the chance of that? and What are the chances of that? – FumbleFingers Jan 16 at 19:06
  • A mathematician might say "The overall probability of winning the lottery is point zero eight," but people don't often talk like that. – Weather Vane Jan 16 at 19:11
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This is how I think of these phrases. The plural is used in "chances/odds are" because a chance of something happening means a possibility of that thing happening. When we say "This year the Cubs have a good chance of winning the championship." or "There's a good chance he is going to be late, given his track record." we are describing the possibility of a single event.

By the same token, as FumbleFingers has shown in this answer, the locution "the chance of doing something" is more common than "the chances of doing something", though the latter is also an idiomatic phrase.

So when we say "The chances are the plane will be grounded for another week." we are talking about all the situations in which this will happen, all the possibilities that will lead to this happening. The same goes for "odds".

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