BE 2100 : FINAL EXAM
1. A Brinell hardness test involves measuring the diameter of the indentation made
when a hardened steel ball is pressed into material under a standard test load. Suppose
that the Brinell hardness is determined for each specimen in a sample size of 81. The
sample's mean hardness was found to be 64.3 with a sample standard deviation of 6.0.
Calculate a 96% confidence interval on the true average Brinell hardness for material
specimens of this type.
a) [61.93 , 66.67] b) [ 62.93 , 65.67] c) [63.60 , 64.98] d) [63.93 , 64.67]
e) [ 64.23, 64.37]
2. Consider the following sample of fat content (in percentage) of n=10 randomly
selected hot dogs.
25.2 21.3 22.8 17.0 29.8 21.0 25.5 16.0 20.9 19.5
Assuming that these were selected from a normal population distribution, construct a
90% confidence interval of the population mean fat content.
a) [18.95 , 24.85] b) 19.21 , 23.99] c) [19.51 , 24.29] d) [20.21 , 22.99]
e) [20.71 , 22.49]
3. Ten percent of the households of a Midwestern suburb for a total of 539 households
were surveyed. Of this total of 539 households, it was found that 133 of these
households owned at least one firearm. Construct a 98% confidence interval on the
proportion of all households in this suburb that own a firearm.
a)[.143 , .350] b) [.173 , .320] c) [.203 , .290] d) [.243 , .438]
e) [ .292 , .388]
4. The accompanying data present 17 data points on breakdown voltage of electrically
stressed flexible printed wiring boards, The data are assumed to be normally
distributed.
1470 1510 1690 1740 1900 2000 2030 2100 2190 2200 2290 2380
2390 2480 2500 2580 2700
Construct a 95% confidence interval on the "standard deviation" of the breakdown
voltage distribution.
a) [251 , 513] b) [261 , 543] c) [271 , 584] d) [301 , 554]
e) [321 , 465]
5. We desire to calculate the average stray load loss in watts for a certain type of
induction motor when the line current is held at 10 amps for a speed of 1500 rpm.
Assume the stray loss is normally distributed with a standard deviation of 3.0 watts.
How large a sample is needed to create a 95% confidence interval to within 0.5 watts
of the true mean?
a) 67 b) 139 c) 196 d) 246 e) 398
6. Ionizing radiation is being given increasing attention as a method for preserving
horticultural products. The process was tested on garlic bulbs. Of 180 that received
radiation 153 were still marketable after 240 days. Of 180 untreated bulbs only 119
