A mean lead level of 15 parts per billion (ppb) is considered safe, while anything above 15 ppb is considered dangerous. Mr. Wilcox believes the Flint water is unsafe, so he takes a random sample of 200 households. He finds the average lead level of the sample is 16.5 ppb, with a standard deviation of 8 ppb (sample standard deviation, not population standard deviation. what is the population proportion