Question

When circuit boards used in the manufacture of compact disc players are tested, the long-run percentage of defectives is 10%. Let X = the number of defective boards in a random sample of size n = 20, so X ~ Bin(20, 0.1). (Round your probabilities to three decimal places.)

(a) Determine P(X ≤ 2).


(b) Determine P(X ≥ 5).


(c) Determine P(1 ≤ X ≤ 4).


(d) What is the probability that none of the 20 boards is defective?


(e) Calculate the expected value and standard deviation of X. (Round your standard deviation to two decimal places.)

expected value	boards
standard deviation	boards

Answer 1

X ~ Bin(20, 0.1)

(a) Determine P(X ≤ 2).

P(X ≤ 2) = P(X=0) + P(X=1) + P(X=2)

(b) Determine P(X ≥ 5 ) = 1 - P(X ≤ 4)

P(X ≥ 5 ) = 1 - 0.9568

P(X ≥ 5 ) = 0.0432

(c) Determine P(1 ≤ X ≤ 4).

(d) What is the probability that none of the 20 boards is defective?

P(X = 0 )

(e) Calculate the expected value and standard deviation of X.

expected value =np = 20*0.1 = 2

standard deviation = sqrt(np(1-p)) = sqrt(20*0.1*0.9) = sqrt(1.8) = 1.34