A sample of 20 diodes is chosen for life testing. The time to failure of the diodes is exponentially distributed. The test is terminated after six failures, with no replacement of the failed items. The failure times (in hours), of the six diodes are 530,590, 670,700,720, and 780. Estimate the mean time to failure of the diodes as well as the failure rate. Find a 95% confidence interval for the mean life.