1. A sample of 100 bean cans showed an average weight of 13 ounces with a standard deviation of 0.8 ounces. Construct a 90% confidence interval for the mean of the population.

Solution: By definition, a 90% confidence interval for the mean of the population is given by

where corresponds to the sample mean, corresponds to the standard deviation, corresponds to the sample size and corresponds to the cutoff point to the standard normal distribution. Since we want 90% confidence interval, we take, and then we look up the normal distribution table to get that. Replacing all this information in the equation we obtain: