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

$12$ units were one failure at $10,000$ hours, two at $25,000$ hours, and two 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$)$ MTBF at this time in testing $=,$ hours