You may need to use the appropriate appendix table or technology to answer this question. The average monthly electric bill of a random sample of 256 residents of a city is $117 with a standard deviation of $34. (a) Construct a 90% confidence interval for the mean monthly electric bills of all residents (in dollars). (Round your answers to the nearest cent.) to $ $ (b) Construct a 95% confidence interval for the mean monthly electric bills of all residents (in dollars). (Round your answers to the nearest cent.) to $