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