Question

The average playing time of compact discs in a large collection is 35 minutes, and the standard deviation is 5 minutes.
a. What value is 1 standard deviation above the mean? 1 standard deviation below the mean? What values are 2 standard deviations away from the mean?
b. Without assuming anything about the distribution of times, at least what percentage of the times is between 25 and 45 minutes?
c. Without assuming anything about the distribution of times, what can be said about the percentage of times that are either less than 20 minutes or greater than 50 minutes?
d. Assuming that the distribution of times is approximately normal, about what percentage of times are between 25 and 45 minutes? less than 20 minutes or greater than 50 minutes? less than 20 minutes?

Answer 1

68% are within 30 minutes to 40 minutes, 95% are within 25 minutes to 45 minutes and 99.7% are within 20 minutes to 50 minutes

The three sigma rule states that 68% are within one standard deviation of the mean, 95% are within two standard deviation from the mean and 99.7% are within three standard deviation from the mean.

Given that μ = 35, σ = 5

68% are within one standard deviation = μ ± σ = 35 ± 5 = (30, 40)

95% are within two standard deviation = μ ± 2σ = 35 ± 2*5 = (25, 45)

99.7% are within three standard deviation = μ ± 3σ = 35 ± 3*5 = (20, 50)

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