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Probability- counting help!!?

Working on the counting section of the probabilty chapter. Ive gotten through a few practice problems but some just make me confused. >>Age Discrimination. The Pitt software company reduced its sales staff from 32 employees to 28 employees. The company claimed that four employees were randomly selected for job termination. However, the four employees chosen are the four oldest employees among the original sales force of 32. Find the probabilty that when four employees are randomly selected from a group of 32, the four oldest are selectd. Is that probabilty low enough to charge that instead of using random selection the PItt software compay actually fired the oldest employees? >>Selection of Treatment group. Walton Pharmaceuticals wants to test the effectiveness of a new drug designed to relieve allergy symptoms. The inital test will be conducted by treating six people chosen from a pool of 15 volunteers. If the treatment group is randomly selected what is the probability that it consists of the six youngest people in the pool? If the six youngest are selected, is there sufficient evidence to conclude that instead of being random, the selection was based on age? For both problems I tried nCr & nPr but neither gave the same result as the back of the book. What am I doing wrong? If neither formula works (nCr & nPr) then what do I do? Any tips on knowing which one to use? Any tips on knowning which vaule is "n" & which value is "r"? (ive just been going with whatever FEELS best but I should probably use a more solid method) Thanks!

Public Comments

1. There are 32C4 = 35960 ways to select 4 people out of 32. Of those, there is only one way to choose the four oldest. That would mean that the probability that the four oldest would be selected is 1/35960. You be the judge. The other one is similar - there is only one way to choose the six youngest people out of 15C6. Hope this helps. ♣ ♣