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.
The probability of a favourable outcome among all possibilities can be expressed: probability (p) equals the total number of favourable outcomes (f) divided by the total number of possibilities (t), or p = f/t.