Joint probability

A joint probability describes the intersection $P(A \cap B)$, so both conditions have to be met by the same individual or trial. In a two-way table you find it by dividing an interior cell count by the grand total, never by a row or column total. Among 200 surveyed students, 45 are seniors who drive to school, so $P(\text{senior and drives}) = 45/200 = 0.225$. Adding all the joint probabilities across a row gives the marginal probability for that row.