
Concept explainers
(a)
Find the z interval for 4.5<x.
(a)

Answer to Problem 11P
The z interval for 4.5<x is z>−1.00.
Explanation of Solution
Calculation:
Z score:
The number of standard deviations the original measurement x is from the value of mean μ is measured using the z-score or z value. The formula for z score is,
z=x−μσ
In the formula, x is the raw score, μ is the mean and σ is the standard deviation.
The variable x is red blood cell (RBC) count in millions per cubic millimetre for women. The healthy females are
z=x−4.80.3
For the z interval consider,
4.5<xx>4.5
Subtract 4.8 on both sides of the inequality.
x−4.8>4.5−4.8x−4.8>−0.3
Divide 0.3 on both sides of the inequality.
x−4.80.3>−0.30.3z>−1.00
Hence, the z interval for 4.5<x is z>−1.00.
(b)
Find the z interval for x<4.2.
(b)

Answer to Problem 11P
The z interval for x<4.2 is z<−2.00.
Explanation of Solution
Calculation:
For the z interval consider,
x<4.2
Subtract 4.8 on both sides of the inequality.
x−4.8<4.2−4.8x−4.8<−0.6
Divide 0.3 on both sides of the inequality.
x−4.80.3<−0.60.3z<−2.00
Hence, the z interval for x<4.2 is z<−2.00.
(c)
Find the z interval for 4.0<x<5.5.
(c)

Answer to Problem 11P
The z interval for 4.0<x<5.5 is −2.67<z<2.33.
Explanation of Solution
Calculation:
For the z interval consider,
4.0<x<5.5
Subtract 4.8 for each part of the inequality.
4.0−4.8<x−4.8<5.5−4.8−0.8<x−4.8<0.7
Divide 4.3 for each part of the inequality.
−0.80.3<x−4.80.3<0.70.3−2.67<z<2.33
Hence, the z interval for 4.0<x<5.5 is −2.67<z<2.33.
(d)
Find the x interval for z<−1.44.
(d)

Answer to Problem 11P
The x interval for z<−1.44 is x<4.4.
Explanation of Solution
Calculation:
The z score is,
z=x−4.80.3x=4.8+0.3z
For the x interval consider,
z<−1.44
Multiply 0.3 on both sides of the inequality.
0.3z<−1.44(0.3)0.3z<−0.432
Add 4.8 on both sides of the inequality.
4.8+0.3z<−0.432+4.8x<4.4
Hence, the x interval for z<−1.44 is x<4.4.
(e)
Find the x interval for 1.28<z.
(e)

Answer to Problem 11P
The x interval for 1.28<z is x>5.2.
Explanation of Solution
Calculation:
For the x interval consider,
1.28<zz>1.28
Multiply 0.3 on both sides of the inequality.
0.3z>1.28(0.3)0.3z>0.384
Add 4.8 on both sides of the inequality.
4.8+0.3z>0.384+4.8x>5.2
Hence, the x interval for 1.28<z is x>5.2.
(f)
Find the x interval for −2.25<z<−1.00.
(f)

Answer to Problem 11P
The x interval for −2.25<z<−1.00 is 4.12<x<4.50.
Explanation of Solution
Calculation:
For the x interval consider,
−2.25<z<−1.00
Multiply 0.3 for each part of the inequality.
−2.25(0.3)<0.3z<−1.00(0.3)−0.675<0.3z<−0.300
Add 4.8 for each part of the inequality.
−0.675+4.8<4.8+0.3z<−0.300+4.84.12<x<4.50
Hence, the x interval for −2.25<z<−1.00 is 4.12<x<4.50.
(g)
Identify whether the RBC count of 5.9 or higher can be considered unusually high or not using z values and Figure 6-15.
(g)

Answer to Problem 11P
The RBC count of 5.9 or higher is unusually high.
Explanation of Solution
Calculation:
The RBC count of 5.9 or higher, that is x≥5.9. Then the value of z is,
x≥5.9
Subtract 4.8 on both sides of the inequality.
x−4.8≥5.9−4.8x−4.8≥1.1
Divide 0.3 on both sides of the inequality.
x−4.80.3≥1.10.3z≥3.67
The RBC count of 5.9 or higher is 3.67 standard deviations above the mean value. The z score value is greater than 3 indicating that the value is very unusual.
The figure 6-15 is the standard normal distribution curve. The z value is located on the curve as below.
The z value that is far from the mean (zero) is considered as unusual. If the value is closer to –3 is usually very small and value closer to 3 is usually very large.
If the value of z for RBC count of 5.9 or higher is greater to 3 then RBC count would be very large and unusual.
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Chapter 6 Solutions
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