Plastic sheets produced by a machine are periodically monitored for possible fluctuations in thickness. If the true variance exceeds 2.15 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 fesults shown below (in millimeters). 229230227230225230230228226225 a. Find the sample variance. b. Test, at the 5% significance level, the null hypothesis that the population variance is at most 2.15 square 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.)