Samples of final examination scores for two statistics classes with different instructors provided the following results: for instructor "A" the sample size was 12, the sample mean score was 72 and the sample standard deviation was 8. For instructor "B" the sample size was 15, the sample mean score was 78 and the sample standard deviation was 10. at a 5% level of significance, test whether these data are sufficient to conclude that the mean grades for the two classes are the same ?