A)
B) s = 304.2
C)
To find the sample mean, add all of the data points together and then divide by 12, the number of data points:
(1425+1227+989+697+729+839+729+742+550+626+1449+941)/12 = 10943/12 = 911.9
To find the sample standard deviation, subtract each data point from the mean; square the result; find the average of these; then take the square root.
To construct a confidence interval, first find the z-score that goes along with it:
Convert 95% to a decimal: 0.95
Subtract from 1: 1-0.95 = 0.05
Divide by 2: 0.05/2 = 0.025
Subtract from 1: 1-0.025 = 0.975
Using a z-table (http://www.z-table.com) we see that this value corresponds with a z-score of 1.96.
We use this to calculate the margin of error:
This gives us the confidence interval