Plastic sheets produced by a machine are periodically monitored for possible fluctuations in thickness. If the true variance exceeds 1.40 square millimeters, there is cause for concern about product quality. Thickness measurements for a random sample of 10 sheets produced in a particular shift were taken, giving the results shown below (in millimeters). 229 228 226 225 229 227 227 226 225 226 a. Find the sample variance. b. Test, at the 10% significance level, the null hypothesis that the population variance is at most 1.40square millimeters. Click the icon to view the upper critical values of Chi-square distribution table. a. The sample variance is square millimeters. (Round to three decimal places as needed.)