A manufacturer of disk drives for notebook computers wants a MTBF of at least $50,000$ hours. Recent test results for

$12$ units were two failures at $10,000$ hours, one at $20,000$ hours, and one more at $40,000$ hours. The remaining units

were still running at $60,000$ hours.

Determine the following:

a$)$ Percent of failures $=,\%($enter your response as a percentage rounded to one decimal place$).$

b$)$ Number of failures per unit$-$hour of operating time $=$ failures per hour $($round your response to eight

decimal places$).$

c$)$ MTBF at this time in testing $=$

hours $($round your response to the nearest whole number$).$