The theoretical definition of probability states that if the outcomes of an event are mutually exclusive and equally likely to happen, then the probability of the outcome "A" is: P(A) = Number of outcomes that favors A / Total number of outcomes. For example, there are two possible outcomes when a coin is tossed in the air, and the probability of the coin landing on a head or a tail is equal to 0.5.

**Identify the number of possible outcomes**For any event, all the possible outcomes must be identified first. These should contain all the possible outcomes in all possible combinations. For instance, if two coins are tossed in the air at the same time then the possible outcomes would be: HH, TH, HT and TT, where "H" represents a head and "T" represents a tail.

**Identify the number of favorable outcomes**Once all the possible outcomes of an event are identified, the number of favorable outcomes that satisfy a specific condition must identified. For example, if two coins are tossed in the air at the same time, the number of outcomes that satisfy the condition of a coin landing on heads at least once is 3.

**Calculate the probability**Once all the numbers are obtained, calculate the probability. For example, the probability of getting at least one head when both coins are tossed in the air at the same time is: P(Head) = 3/4 = 0.75.