Assume that an urn contains formula_1 red balls and formula_2 white balls, totalling formula_3 balls. formula_4 balls are drawn at random from the urn one by one without replacement. Each red ball has the weight formula_5, and each white ball has the weight formula_6. We assume that the probability of taking a particular ball is proportional to its weight. The physical property that determines the odds may be something else than weight, such as size or slipperiness or some other factor, but it is convenient to use the word "weight" for the odds parameter.