Respuesta :
Answer:
You get a D if you have a grade between 66 and 72.
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.
Scores on the test are normally distributed with a mean of 79.3 and a standard deviation of 8.4.
This means that [tex]\mu = 79.3, \sigma = 8.4[/tex]
D: Scores below the top 80% and above the bottom 6%
So between the 6th and the 20th percentile.
6th percentile:
X when Z has a pvalue of 0.06. So X when Z = -1.555.
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]-1.555 = \frac{X - 79.3}{8.4}[/tex]
[tex]X - 79.3 = -1.555*8.4[/tex]
[tex]X = 66.2[/tex]
Rounds to 66
20th percentile:
X when Z has a pvalue of 0.2. So X when Z = -0.84.
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]-0.84 = \frac{X - 79.3}{8.4}[/tex]
[tex]X - 79.3 = -0.84*8.4[/tex]
[tex]X = 72.2[/tex]
Rounds to 72
You get a D if you have a grade between 66 and 72.
