An electrical engineer wishes to compare the mean life times of two types of transistors in an application involving high temperature performance. a sample of 60 transistors of type a were tested and were found to have a mean lifetime of 1827 hours and a standard deviation of 168 hours. a sample of 180 transistors of type b were tested and were found to have a mean lifetime of 1658 hours and a standard deviation of 225 hours. can we reject, at the 5% level of significance, the hypothesis of equal mean