The game has 3 reels. Each reel has 10 symbols on it, and there’s an equal
chance of getting any symbol. The probability of getting 3 of the same symbols
in a row is the product of the probability of getting that symbol on each reel.
The probability is 1/10 X 1/10 X 1/10 = 1/1000 on any combination.
If one of those combinations pays off at 900 for 1, it’s easy to see how the
casino stands to make a profit on that wager. You have a 999 to 1 chance of
winning, but you only win 899 units.