units were one failure at $10,000$ hours, one more 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$)$ Percent of failures $=40\%($enter your response as a percentage rounded to one decimal place$).$

b$)$ Number of failures per unit$-$hour of operating time $=0\mathrm{.}00000825$ failures per hour $($round yo$$r response to eight decimal places$).$

c$)$ MTBF at this time in testing $=$ hours $($round your response to the nearest whole number$).$