Respuesta :
Answer:
The standard deviation is $13,052.
Step-by-step explanation:
Normal Probability Distribution:
Problems of normal distributions can be solved using the z-score formula.
In a set with mean [tex]\mu[/tex] and standard deviation [tex]\sigma[/tex], the zscore of a measure X is given by:
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.
When the head of the household has a college degree, the mean before-tax family income is $ 85,050.
This means that [tex]\mu = 85,050[/tex]
Suppose that 56% of the before-tax family incomes when the head of the household has a college degree are between $75,000 and $95,100 and that these incomes are normally distributed.
They are equally as far from the mean, one above, and one below. This means that when [tex]X = 95100[/tex], Z has a pvalue of 0.5 + (0.56/2) = 0.78. So when X = 95100, Z = 0.77. We use this to find [tex]\sigma[/tex].
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]0.77 = \frac{95100 - 85050}{\sigma}[/tex]
[tex]0.77\sigma = 10050[/tex]
[tex]\sigma = \frac{10050}{0.77}[/tex]
[tex]\sigma = 13052[/tex]
The standard deviation is $13,052.
