Answer:
a) 
b) The 95% confidence interval is given by (8.197;9.943)
c) 
d) 3 possible ways
1) Increasing the sample size n.
2) Reducing the variability. If we have more data probably we will have less variation.
3) Lower the confidence level. Because if we have lower confidence then the quantile from the t distribution would belower and tthe margin of error too.
Step-by-step explanation:
Notation and definitions
n=15 represent the sample size
represent the sample mean
represent the sample standard deviation
m represent the margin of error
Confidence =95% or 0.95
A confidence interval is "a range of values that’s likely to include a population value with a certain degree of confidence. It is often expressed a % whereby a population means lies between an upper and lower interval".
The margin of error is the range of values below and above the sample statistic in a confidence interval.
Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".
Part a: Find the point estimate of the population mean.
The point of estimate for the population mean 
is given by:
The mean obteained after add all the data and divide by 15 is
Calculate the critical value tc
In order to find the critical value is important to mention that we don't know about the population standard deviation, so on this case we need to use the t distribution. Since our interval is at 95% of confidence, our significance level would be given by 
and 
. The degrees of freedom are given by:
We can find the critical values in excel using the following formulas:
"=T.INV(0.025,14)" for 
"=T.INV(1-0.025,14)" for 
The critical value 
Part c: Calculate the margin of error (m)
First we need to calculate the standard deviation given by this formula:
s=1.580
The margin of error for the sample mean is given by this formula:
Part b: Calculate the confidence interval
The interval for the mean is given by this formula:
And calculating the limits we got:
The 95% confidence interval is given by (8.197;9.943)
Part d: How can we reduce the margin of error?
We can reduce the margin of error on the following ways:
1) Increasing the sample size n.
2) Reducing the variability. If we have more data probably we will have less variation.
3) Lower the confidence level. Because if we have lower confidence then the quantile from the t distribution would belower and tthe margin of error too.
