Let S measure the time-dependent strength of a material under test and So 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. S2 = (S0)2 - 2Ct (t = time) 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?