Balls fall into a grid of rc boxes arranged in r rows and c columns. Suppose the probability of a ball falling into a box in row i is pi , and the probability of a ball falling into a box in column j is qj , for i = 1, . . . , r, j = 1, . . . , c. If the probability of falling into a row is independent of the column, the probability a particular ball lands in the box in row i, column j is