An urn contains n red balls and m blue balls. They are withdrawn one at a time until a total of r, r ≤ n, red balls have been withdrawn. Find the probability that a total of k balls are withdrawn. Hint: A total of k balls will be withdrawn if there are r − 1 red balls in the first k − 1 withdrawals and the kth withdrawal is a red ball.

Bella want to know what the students at her school think of her show. which is the best group to survey to find this information a. go to the mall and ask all of the students she recognizes from school b. ask her viewers to call in durning an episode of the show c. get a list of students at her school from the office and use a random number generator to pick 30 of them d. get a list of students at her school from the office and pick the first 30 students on the list