<PRE><B>Question 998759</B>
The proportion in the sample was 0.8
The SE is sqrt ((0.8)(0.2/2395}=0.00817
a 95% CI is 1.96*SE for the interval half width.
It equals 0.016
The CI is (0.784,0.816)
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Telephone
0.5298 is point estimate proportion
SE={0.5298*0.4702/2395}^(1/2)=0.0102
1.96*SE=0.02
The CI is (0.51,0.53).  More decimal places may be added if desired.  The CI is wider, because the proportion is closer to 0.5
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sample size unknown
we know that 1.96 * SE is the interval, and the interval must be +/- 20 hours
The SE is the SD/sqrt(n)
1.96*SD/sqrt(n)=20
square both sides
3.84*10000/n =400
n=38416/400=96.04 round to 97
Check this
mean +/-  s/sqrt(n)
that is 100/sqrt(97)=10.153
That *1.96=19.90, slightly below 20 because need to round the 96.04 up to next higher.
97 light bulbs.
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