Problem 4 Computer keyboard failures can be attributed to electrical defects or

Question

Problem 4 Computer keyboard failures can be attributed to electrical defects or mechanical defects. A repair facility currently has 25 failed keyboards, 7 of which have electrical defects and 18 of which have mechanical defects a) In how many ways can a sample of 7 keyboards be selected so that exactly two have an electrical defect? b) If a sample of 7 keyboards is randomly selected, what is the probability that at least 6 of these will have a mechanical defect? (Round your answer to four decimal places.)

Explanation / Answer

a) No of ways = 7C2 * 18C5

                       = 7!/(2! * 5!) * 18!/(5! * 13!)

                       = 179928

b) Probability = (18C6 * 7C1 + 18C7)/25C7

                      = ((18!/(6! * 12!) * 7!/(1! * 6!)) + 18!/(7! * 11!))/(25!/(7! * 18!))

                       = 0.3365

                     

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