A manufacturer of disk drives for notebook computers

wants an MTBF of at least $50,000$ hours. Recent test results for

$17\mathrm{.}7$ What is the overall reliability that bank loans will be

processed accurately if each of the five clerks shown in the chart

has the reliability shown?

$10$ units were one failure at $10,000$ hours, another at $25,000$ hours,

and two more at $45,000$ hours. The remaining units were still running

at $60,000$ hours. Determine the following:

a$)$ Percentage of failures

b$)$ Number of failures per unit$-$hour of operating time

c$)$ MTBF at this point in the testing