Twelve identical products were put on test for 65 days, by which time 9 had failed at 21, 25, 29, 33, 38, 43, 49, 56 and

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Twelve identical products were put on test for 65 days, by which
time 9 had failed at 21, 25, 29, 33, 38, 43, 49, 56 and 65 days
whilst the remaining 3 were still fully functional when the test
was ended. Assuming constant failure rate,
1) Calculate the MTTF of the product;
2) Estimate its reliability at 40 days;
3) Comment on whether the assumption of constant failure rate is
justified.