PLAY SLOTS FOR PROFIT, NOT AS A JOB

The machine’s most frequent payout is for two horseshoes with something other than a star on the third reel, which pays out two coins when it occurs and should occur 200 times for every 1,000 plays. We pick 1,000 plays because there are 1,000 possible combinations on the three reels (10 X 10 X 10).
That 200 figure comes from 5 (the number of times the horseshoe appears on the first reel) times 5 (the number of times the horseshoe appears on the second reel) times 8. The 8, in turn, comes from the 10 spots on the third reel minus the 2 bells, which, as we shall see, pay more.
So over the course of 1,000 plays, the machine pays out 400 coins because just two horseshoes come up. If that third reel does come up a star, the machine pays off four coins. Out of 1,000 plays, the combination of two horseshoes and a star is expected to come up 50 times (5X5X2; the 2 represents the 10 positions on the third reel minus the 8 positions that don’t have bells on them) for a total payoff of 200 coins.

To see how this works, let’s take a look at a very simple early slot machine, one with three reels and 10 symbols on each reel. The first two reels are identical. Each has five horseshoes, two spades, one diamond, one heart, and a bell. The third reel is different. It has two stars, two spades, three diamonds, one heart, and two bells.