Suppose that 30% of all telephone calls at work are for personal reasons. If a random sample of 15 is taken, let X be the number of personal calls. The standard deviation of the # of personal calls is:

Answer 1

Standard deviation = √npq

n = sample size = 15
p = .30
q = 1 - p = .70

I'll let you take it from here.

Answer 2

A telephone exchange operator assumes that 7% of the phone calls are wrong numbers.
If the operator is correct, what is the probability that the proportion of wrong numbers in a sample of 624 phone calls would differ from the population proportion by greater than 3% ? Round your answer to four decimal places.