Amazon.com is testing the use of drones to deliver packages for same-day delivery. In order to quote narrow time windows, the variability in delivery times must be sufficiently small. Consider a sample of 24 drone deliveries with a sample variance of s² = .81. 

a. Construct a 90% confidence interval estimate of the population variance for the drone delivery time. 

b. Construct a 90% confidence interval estimate of the population standard deviation.