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The resting heart rates for a sample of individuals are normally distributed with a mean of 70 and a standard deviation of 7. Use the 68-95-99.7 rule to find the percentage of heart rates in each of the following categories.

a. Greater than 77
b. Less than 56
c. Less than 77
d. Less than 84
e. Greater than 70
f. Less than 63
g. Greater than 56
h. Between 63 and 84

Answer :

a. Greater than 77: [tex]\( 16\% \)[/tex]

b. Less than 56: [tex]\( 2.5\% \)[/tex]

c. Less than 77: [tex]\( 84\% \)[/tex]

d. Less than 84: [tex]\( 97.5\% \)[/tex]

e. Greater than 70: [tex]\( 50\% \)[/tex]

f. Less than 63: [tex]\( 15.87\% \)[/tex]

g. Greater than 56: [tex]\( 97.5\% \)[/tex] (complement of less than 56)

h. Between 63 and 84: [tex]\( 81.63\% \)[/tex]

To solve these problems using the 68-95-99.7 rule for a normal distribution with mean [tex]\(\mu = 70\)[/tex] and standard deviation [tex]\(\sigma = 7\)[/tex] we will calculate the percentages step by step.

Step-by-step calculations:

a. Greater than 77:

Calculate the z-score:

[tex]\[ z = \frac{77 - 70}{7} = 1 \][/tex]

The percentage of heart rates greater than 77 is the area to the right of [tex]\( z = 1 \)[/tex] on the standard normal distribution.

[tex]\[ \text{Percentage} = 100\% - \text{percentage within } z = 1 \text{ (which is } 84\%\text{)} \][/tex]

[tex]\[ \text{Percentage} = 100\% - 84\% = 16\% \][/tex]

b. Less than 56:

Calculate the z-score:

[tex]\[ z = \frac{56 - 70}{7} = -2 \][/tex]

The percentage of heart rates less than 56 is the area to the left of [tex]\( z = -2 \)[/tex] on the standard normal distribution.

[tex]\[ \text{Percentage} = \text{percentage within } z = -2 \text{ (which is } 2.5\%\text{)} \][/tex]

c. Less than 77:

Already calculated in part (a), but for completeness:

[tex]\[ \text{Percentage} = \text{percentage within } z = 1 \text{ (which is } 84\%\text{)} \][/tex]

d. Less than 84:

Calculate the z-score:

[tex]\[ z = \frac{84 - 70}{7} = 2 \][/tex]

The percentage of heart rates less than 84 is the area to the left of[tex]\( z = 2 \)[/tex] on the standard normal distribution.

[tex]\[ \text{Percentage} = \text{percentage within } z = 2 \text{ (which is } 97.5\%\text{)} \][/tex]

e. Greater than 70:

This is [tex]\( 50\% \)[/tex] because the mean divides the normal distribution symmetrically.

f. Less than 63:

Calculate the z-score:

[tex]\[ z = \frac{63 - 70}{7} = -1 \][/tex]

The percentage of heart rates less than 63 is the area to the left of [tex]\( z = -1 \)[/tex] on the standard normal distribution.

[tex]\[ \text{Percentage} = \text{percentage within } z = -1 \text{ (which is } 15.87\%\text{)} \][/tex]

g. Greater than 56:

Already calculated in part (b), but for completeness:

[tex]\[ \text{Percentage} = 100\% - \text{percentage within } z = -2 \text{ (which is } 2.5\%\text{)} \][/tex]

h. Between 63 and 84:

To find the percentage of heart rates between 63 and 84, we calculate the sum of percentages from parts (d) and (f):

[tex]\[ \text{Percentage} = \text{percentage within } z = 2 \text{ (less than 84)} - \text{percentage within } z = -1 \text{ (less than 63)} \[ \text{Percentage} = 97.5\% - 15.87\% \][/tex]

[tex]\[ \text{Percentage} = 81.63\% \][/tex]

These percentages reflect the distribution of heart rates based on the given mean and standard deviation, using the properties of the normal distribution.

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