Let "X=" the number of plasma televisions with defective speakers: "X\\sim Bin(n, p)"
Given "n=400, p=0.1"
Use the dishonest-coin principle with p=0.1 to find
"\\sigma=\\sqrt{np(1-p)}=\\sqrt{400(0.1)(1-0.1)}=6"
"28=40-2\\times6=\\mu-2\\sigma"
By the 68–95–99.7 rule
The probability that there are more than 28 with defective speakers
Or
"n=400\\geq30, np=40\\geq5, n(1-p)=360\\geq5"
Then, "X" has an approximately normal distribution with mean "\\mu=np=40" and standard deviation
"\\sigma=\\sqrt{np(1-p)}=6"
"1-P(Z\\leq{28.5-40\\over6}) \\approx1-0.02764\\approx0.97236"
By Binomial distribution
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