If you want to know what is the probability that the coin will come up heads, then that would be: heads / (heads + tails) = 1/2 = . 5. Likewise when playing an even money bet at roulette, that option covers 18 of the 37 possible outcomes: 18/37=. 48648649.

Martingale is definitely one of the most widely known and most successful roulette strategies (for some it is the best roulette strategy), and it is very simple to apply. It can be used only for outside bets, those that provide almost 50% chance of winning, and excluding the 0.