Question

A sample of the salaries of assistant professors on the business faculty at a local university revealed a mean income of $100,000 with a standard deviation of $10,000. Assume that salaries follow a bell-shaped distribution. Use the empirical rule:

a. Approximately what percentage of the salaries fall between $90,000 and $110,000?
b. Approximately what percentage of the salaries fall between $80,000 and $120,000?
c. Approximately what percentage of the salaries are greater than $120,000?

Answer 1

a) 68%

b) 95%.

c) 2.5%

Explanation:

The Empirical Rule states that, for a normally distributed random variable:

Approximately 68% of the measures are within 1 standard deviation of the mean.

Approximately 95% of the measures are within 2 standard deviations of the mean.

Approximately 99.7% of the measures are within 3 standard deviations of the mean.

In this problem, we have that:

Mean of 100,000, standard deviation of 10,000.

a. Approximately what percentage of the salaries fall between $90,000 and $110,000?

90,000 = 100,000 - 10,000

110,000 = 100,000 + 10,000

Within 1 standard deviation of the mean, so approximately 68%.

b. Approximately what percentage of the salaries fall between $80,000 and $120,000?

80,000 = 100,000 - 2*10,000

120,000 = 100,000 + 2*10,000

Within 2 standard deviations of the mean, so approximately 95%.

c. Approximately what percentage of the salaries are greater than $120,000?

More than 2 standard deviations above the mean.

Approximately 95% of the measures are within 2 standard deviations of the mean, so approximately 5% are more than 2 standard deviations from the mean.

The normal distribution is symmetric, which means that 2.5% are more then 2 standard deviations below the mean, and 2.5% are more than 2 standard deviations above the mean, which means that 2.5% of the salaries are greater than $120,000.