Electrical engineers recognize that high neutral current in computer power syste

Question

Electrical engineers recognize that high neutral current in computer power systems is a potential problem. A recent survey of computer power system load currents at U.S. sites found that 10% of the sites had high neutral current (IEEE Transactions on Industry Applications, July/August. 1990). In a sample of 20 computer power systems selected from the large number of sites in the country, let x be the number with high neutral current.

-Find mean and standard deviation of x

-Find the probability that fewer than two computers have found with high neutral current.

-Find the probability that more than five computers have found with high neutral current.

Explanation / Answer

a)

mean = 20 * 10% = 2

standard deviation = sqrt(20 * 10% * 90%) = 1.342

b)

P(X < 2) = P(X=0) + P(X=1)

= 20C0 * 0.9^20 + 20C1 * 0.1 *0.9^19

= 0.3917

c)

P(X > 5) = 1- P(X<=5)

= 1 - 0.9888

= 0.0112

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