Which expression correctly represents P[x], the probability of outcome x?

• A. The total number of outcomes divided by the number of ways x can occur
• B. The square of the number of ways x can occur
• C. The number of ways x can occur divided by the total number of possible outcomes
• D. The probability of all outcomes summed

Answer: (C)

Probability is the ratio of how many ways a specific outcome can occur to how many total outcomes are possible, assuming each outcome is equally likely. So P[x] is the number of outcomes that correspond to x divided by the total number of possible outcomes in the sample space. For a fair six-sided die, for example, there is one favorable outcome for getting a 3 out of six possible outcomes, so P[3] = 1/6. The other expressions don’t fit: swapping numerator and denominator would invert the probability, squaring the count isn’t how probabilities are formed, and summing the probabilities of all outcomes would give 1, not the probability of a single outcome.