# Having trouble w/ understanding pdf...

by semidevil
Tags: trouble, w or
 Sci Advisor HW Helper P: 2,586 When you say, "the formula is ..." doesn't it strike you as odd that there is no "=" sign. What is a formula without that sign? What you have is an expression for the probability of getting k of the things which there are r of in total. If "r" represents the number of red balls in total, and "w" represents the number of white balls, then: $$P(k) = \frac{{r\choose k} {w\choose {n - k}}}{{N\choose n}}$$ However, this only holds for max{0, n – w} ≤ k ≤ min{n, r}. Clearly, the probability is zero otherwise.