the answer to your third problem is: c
A new paper calls 20 of its subscribers to determine their mean income. The distribution of the sample mean is:
a. unknown
b. binomial
c. normal
d. approximately normal
A sample of 25 is taken from a population which has a normal distribution with a mean of 60 and standard deviation of 15. The probability that the sample mean is between 55.5 and 64.5 is:
a. .0668
b. .2668
c. .5468
d. .8664
e. .9332
Decreasing the sample size:
a. decrease the sample mean
b. decrease the standard deviation of the sample mean
c. increase the standard deviation of the sample mean
d. increase the sample mean