All slot machines are built to take in more than they give back, but buyers of the machine can request (within limits) that the machine always retain a certain percentage.
In other words, when ordering a particular model, the operator can request a “tight” or a “liberal” machine. There is a recent trend toward liberal machines because hard core slot machine players know which machines are generous; they simply won’t play on the mean machines, and word spreads.
The slot machines in Las Vegas and Reno are reputed to be among the most liberal anywhere in the world, but the generous amount the machines return is compensated by the high volume of gamblers attracted to the machines in those cities.
The standard slot machine has three reels. Each reel tape contains 20 symbols, the famous oranges, cherries, and the like. To win, of course, one must get 3 of the same symbol across the board. But since there are 20 symbols on three tapes, there are 8,000 possible combinations (20 x 20 x 20).
By use of simple mathematics and sophisticated electronics, the machine is programmed to retain a certain percentage of money over this cycle of 8,000, and to give back a certain percentage. Slot machines worldwide generally hold from 3 to 20 percent of the intake and give back 80 to 97 percent.
Not every player who puts $100 into a 90 percent machine will get $90 back, of course; success lies in hitting the cycle and the right moment.