Answer:
a) 0.18% of cold sufferers experience fewer than 4 days of symptoms
b) 64.40% of cold sufferers experience symptoms for between 7 and 10 days.
Step-by-step explanation:
Problems of normally distributed samples are solved using the z-score formula.
In a set with mean
and standard deviation
, the zscore of a measure X is given by:
![Z = \frac{X - \mu}{\sigma}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7BX%20-%20%5Cmu%7D%7B%5Csigma%7D)
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.
In this problem, we have that:
![\mu = 7.5, \sigma = 1.2](https://tex.z-dn.net/?f=%5Cmu%20%3D%207.5%2C%20%5Csigma%20%3D%201.2)
a. What proportion of cold sufferers experience fewer than 4 days of symptoms?
This is the pvalue of Z when X = 4. So
![Z = \frac{X - \mu}{\sigma}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7BX%20-%20%5Cmu%7D%7B%5Csigma%7D)
![Z = \frac{4 - 7.5}{1.2}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7B4%20-%207.5%7D%7B1.2%7D)
![Z = -2.92](https://tex.z-dn.net/?f=Z%20%3D%20-2.92)
has a pvalue of 0.0018.
So 0.18% of cold sufferers experience fewer than 4 days of symptoms.
b. What proportion of cold sufferers experience symptoms for between 7 and 10 days?
This is the pvalue of Z when X = 10 subtracted by the pvalue of Z when X = 7. So
X = 10
![Z = \frac{X - \mu}{\sigma}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7BX%20-%20%5Cmu%7D%7B%5Csigma%7D)
![Z = \frac{10 - 7.5}{1.2}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7B10%20-%207.5%7D%7B1.2%7D)
![Z = 2.08](https://tex.z-dn.net/?f=Z%20%3D%202.08)
has a pvalue of 0.9812.
X = 7
![Z = \frac{X - \mu}{\sigma}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7BX%20-%20%5Cmu%7D%7B%5Csigma%7D)
![Z = \frac{7 - 7.5}{1.2}](https://tex.z-dn.net/?f=Z%20%3D%20%5Cfrac%7B7%20-%207.5%7D%7B1.2%7D)
![Z = -0.42](https://tex.z-dn.net/?f=Z%20%3D%20-0.42)
has a pvalue of 0.3372.
So 0.9812 - 0.3372 = 0.644 = 64.40% of cold sufferers experience symptoms for between 7 and 10 days.