A study of 420,097 cell phones users found that 137 of them developed cancer of the brain or nervous system. Prior to this study of cell phone use, the rate of such cancer was found to be 0.0425% for those not using cell phones.

a. use the sample data to construct a 90% confidence interval estimate of the percentages of cell phone users who develop cancer of the brain or nervous system.

b. Do cell phone users appear to have rate of cancer of the brain or nervous system that is different from the rate of such cancer among those not using cell phones? why or why not?