Respuesta :
Answer:
a) 0.9544 = 95.44% of scores lie between 220 and 380 points.
b) 0.1587 = 15.87% probability that a randomly chosen student scores is below 260.
c) 25.14% of scores are above 326.8 points.
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.
Mean of 300 and a standard deviation of 40.
This means that [tex]\mu = 300, \sigma = 40[/tex]
(a) What proportion of scores lie between 220 and 380 points?
This is the p-value of Z when X = 380 subtracted by the p-value of Z when X = 220.
X = 380
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]Z = \frac{380 - 300}{40}[/tex]
[tex]Z = 2[/tex]
[tex]Z = 2[/tex] has a p-value of 0.9772.
X = 220
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]Z = \frac{220 - 300}{40}[/tex]
[tex]Z = -2[/tex]
[tex]Z = -2[/tex] has a p-value of 0.0228.
0.9772 - 0.0228 = 0.9544
0.9544 = 95.44% of scores lie between 220 and 380 points.
(b) What is the probability that a randomly chosen student scores is below 260?
This is the p-value of Z when X = 260. So
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]Z = \frac{260 - 300}{40}[/tex]
[tex]Z = -1[/tex]
[tex]Z = -1[/tex] has a p-value of 0.1587.
0.1587 = 15.87% probability that a randomly chosen student scores is below 260.
(c) What percent of scores are above 326.8 points?
The proportion is 1 subtracted by the p-value of Z when X = 326.8. So
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]Z = \frac{326.8 - 300}{40}[/tex]
[tex]Z = 0.67[/tex]
[tex]Z = 0.67[/tex] has a p-value of 0.7486.
1 - 0.7486 = 0.2514
0.2514*100% = 25.14%
25.14% of scores are above 326.8 points.
