17.7 A manufacturer of touch screens for tablets wants a MTBF of at least 50 000 hours. Recent test results for 10 units were one failure at 10 000 hrs, another at 25 000 hrs, and two more at 45 000 hrs. The remaining units were still running at 60 000 hours. Determine the following: a) Percentage of failures b) Number of failures per unit-hour c) MTBF at this point in the testing