Computer keyboard failures can be attributed to electrical defects or mechanical

Question

Computer keyboard failures can be attributed to electrical defects or mechanical defects. A repair facility currently has 25 failed keyboards, 12 of which have electrical defects and 13 of which have mechanical defects.

(b)

In how many ways can a sample of 7 keyboards be selected so that exactly two have an electrical defect?

(c)

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

Number of failed keyboards = 25

electrical defects = 12

MEchanical defects = 13

(b) HEre exactly two have an electrical defect.

Number of ways = 12C2 * 13C5 = 66 * 1287 = 84942

(c) Pr ( Mechanical Defect = 6,7) = 12C113C6/25C7 + 12C013C7/25C7 = 12 * 1716/480700 + 1716/480700

= 0.0464

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