Homework 6
Chapter 9
Problem 4
Your quality control department has just analyzed the contents of 20 randomly selected barrels
of materials to be used in manufacturing plastic garden equipment.
The results found an
average of 41.93 gallons of usable 1material per barrel, with a standard error of 0.040 gallon
per barrel.
Find the twosided 95% confidence interval for the population mean.
X
tS
X
±
Xbar = 41.93
tcrit at 95% w/ 19 d.f. = 2.093
S
xbar
= 0.040
(given)
)
040
.
0
(
)
093
.
2
(
93
.
41
×
±
The 95% confidence interval extends from 41.85 to 42.01 gallons.
Problem 16
The following quotes for cleaning cost have been obtained from a random sample of 12
providers chosen from a much larger population, prior to awarding a contract for these
services:
$114, 154, 142, 132, 127, 145, 135, 138, 126, 142, 135, 124
a.
Approximately how far is the average of these 12 quotes from the unknown mean for the
entire population of providers?
Xbar = $134.50
S = $10.79
=
=
=
12
79
.
10
n
S
S
X
$3.115
b.
Find the 95% confidence interval for the population mean quote.
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 Fall '09
 Padiego
 Physics, Statistics, Normal Distribution, Standard Deviation, Work, Mean

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