A two-way table sorts every tray cell by two questions at once, and it is the sample space for every probability this week. An event is a set of cells: a row, a column, an "or" (union), an "and" (intersection) or a "not" (complement). The conditional probability P(A given B) is the fraction of B's cells that are also in A, which is P(A and B) ÷ P(B). Two events are independent when P(A given B) equals P(A), or equally when P(A and B) = P(A) × P(B). The addition rule P(A or B) = P(A) + P(B) - P(A and B) subtracts the overlap so it is counted once.