Let S measure the time-dependent strength of a material under test and S(0)is the initial strength as measured at the beginning of test.Prior tests and evaluations have suggested that the following stress-time relationship appears to work S^4-4Ct .If the strength declines 20 per cent after exactly three weeks of accelerated test, what is the expected time to failure at this condition, when the failure is defined as a decline of strength of 50 per cent?