This is known as the cancellation system. Pick a series of numbers, say 1,2,3,4,5. Your first bet is the total of the two numbers on the end, or 6 units in this case. If you lose it, you cancel out the 1 and the 5, add the six to the end of the series and bet the 6 and the 2, for a total of eight units bet. Lose that one and you add the eight to the series, cancel the 2 and the six and bet a total of 11 (3+8) and so on. If you win, you proceed to the inside numbers until they are all cancelled out. Again, a loser, but an insidious one.

The six and eight can each be made five ways. So if you are up on the don't six, you have six ways to win (on the number seven) and five ways to lose. When the bet is $10 you would win $60, lose $50, and be ahead by $10 after the 11 decisions (on average) If you see a don't player about to take his bet down just say to the fellow: "Don't! Here's 10 bucks, you're even!" He'll take the $10 because that's what his bet is.