Respuesta :
Answer:
Scores between 71 and 80 give a C grade.
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 z-score 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 p-value, 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 78.8 and a standard deviation of 9.8.
This means that [tex]\mu = 78.8, \sigma = 9.8[/tex]
Find the numerical limits for a C grade.
Above the bottom 20%(20th percentile) and below the top 45%(below the 100 - 45 = 55th percentile).
20th percentile:
X when Z has a p-value of 0.2, so X when Z = -0.84.
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]-0.84 = \frac{X - 78.8}{9.8}[/tex]
[tex]X - 78.8 = -0.84*9.8[/tex]
[tex]X = 70.57[/tex]
So it rounds to 71.
55th percentile:
X when Z has a p-value of 0.55, so X when Z = 0.125.
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]0.125 = \frac{X - 78.8}{9.8}[/tex]
[tex]X - 78.8 = 0.125*9.8[/tex]
[tex]X = 80[/tex]
Scores between 71 and 80 give a C grade.
