an analyst from an energy research institute in california wishes to estimate the 90% confidence interval for the average price of unleaded gasoline in the state. in particular, she does not want the sample mean to deviate from the population mean by more than $0.05. what is the minimum number of gas stations that she should include in her sample if she uses the standard deviation estimate of $0.31, as reported in the popular press? (you may find it useful to reference the z table. round up final answer to nearest whole number.)