A manufacturer wants to increase the shelf life of a line of cake mixes. past records indicate that the average shelf life of the mix is 216 days. after a revised mix has been developed, a sample of nine boxes of cake mix gave these shelf lives (in days): 215, 217, 218, 219, 216, 217, 217, 218, and 218. using a= .025, has the shelf life of the cake mix increased?