The odds for each game are stacked in favor of the casino. This means that, the more you play, the more the math works against you, and the better the chances are of you walking out of the casino with less money in your wallet than when you came in.
When it comes to the number 7 and gambling, it is widely believed that it's the fairest number to predict. In other words, the fairest chance of winning and losing is said to be by guessing a number between one and seven.