急！！ 关于“MTBF”的问题请大家帮忙！！！

sar6000

这是一个关于“MTBF”（平均故障时间）计算的问题：有那位大侠知道，多谢请教：
      
>; >; The reliability of a hard-disk drive typically describle in terms of a quantity called mean time between failures(MTBF). Although this quantity is called a "time", the MTBF actually is measured in drive-hours per failure.
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>; >; a. If a system contain 1000 disk drives, each of which has a 750,000 hour MTBF, which of the following best describes how often a drive failure will occur in that disk farm: once per thousand year, once per century, once per decade, once per year, pnce per month, once per week, once per day, once oerhour, once per minute, or once per second?
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>; >; b. Mortality statistics indicate that, on the average, a U.S. resident has about 1:1,000 chance of dying between ages 20 and 21 years. Deduce the MTBF hours for 20 years old. Convert this figure from hours to years. What does this MTBF tell you about the expected lifetime of a 20 year old?
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>; >; c. The manufacturer guarantees a 1-million hour MTBF for a certain model of disk drive. What can you conclude about the number of years for which one fo these drives is under warranty?