## MATH 129 A population has a standard deviaion ? = 21.3.

Question

A population has a standard deviaion ? = 21.3. How large a sample must be drawn so that a 99% confidence interval for ? will have a margin of error equal to 2.5?

The amount of money people spend in a trip to the mall has an unknown distribution with an unknown population mean and a population standard deviation of $15. A random sample of 12 people are surveyed and asked how much money they spent in the mall and gives a sample mean of $250. Find a 90% confidence interval for the population mean.

____ < ? < ____ The number of people who stop for coffee in the morning is normally distributed with an unknown population mean and a population standard deviation of 0.55 times. A random sample of 225 people are surveyed and asked how many times per week they stop for coffee and gives a sample mean of 3.5 times per week. Find a 99% confidence interval for the population mean. _____< ? < ____

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