statistical testing - finding the interval estimate

17) Michele wishes to give a confidence interval for the percentage of Illinois

residents who agree with a certain law. She questions 1500 people from a good

sample and and that 960 agree with the policy. If she chooses a confidence level of

.91, what should her interval estimate for the percentage of people who agree with

this law be? (You may assume that the population of Illinois is infinite.)

A:

I calculated the mean to be .64

The estimated population standard deviation to be .48

The standard deviation to be .4804

When I plug all that in to the formula to get the standard error formula, I get the wrong answer. The teacher says the outcome should be between .61 and .66

18)

(a) What interval estimate for a population parameter M can one get for a confidence interval of .80? (Express the interval in terms of x (the sample mean) and ox (standard error of the sampling distribution.))

A:

I divided 80/2 = 40 which gives a Z value = 1.7

That is all I know for that one.