1) A recent study on the lifetimes of cell phones found that the mean lifetime is 24.3 months and the standard deviation is 2.6 months. A random sample of 20 cell phones is selected. Assuming cell phone lifetimes are normally distributed, what is the probability that the mean lifetime will be longer than 25 months?

2) A random sample of 12 school districts from a moderately populated area showed that the mean number of students per teacher was 19.2 with a standard deviation of 2.1.

a) Construct a 95% confidence interval estimate of the true mean number of students per teacher in this area.

b) Write a sentence interpreting the interval you found in part a.

****PLEASE, show me the formula used and step by step. Thank you.****