Three bells
The most generous payout in the machine is for three bells, which pays 20 coins each time it happens. Since there are two bells on the third reel, along with one apiece on the first two reels, three bells can be expected to occur twice as often as three hearts. The combination can be expected to occur twice per 1,000 plays for a total payout over that period of 40 coins.
Adding up all the payouts, you find that this simple three-reel machine with 10 symbols on each reel pays out only 756 coins for every 1,000 it consumes. That’s a payout rate of 75.6 percent-not very good for the player.
By the mid-1930s machines were already a good deal more complex-but the odds for the player were no better. In fact, they had grown a little worse.