Probability
Probability is favourable outcomes over total outcomes. Add for "or" when events cannot overlap, multiply for "and".
A worked example
Rolling a 2 or a 5: those cannot both happen, so 1/6 + 1/6 = 1/3. Two aces without replacement: (4/52) x (3/51).
The mistake people actually make
Odds compare wins to losses; probability compares wins to the total. Odds of 3:5 means a probability of 3/8, not 3/5.