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A server crashes on the average once in 30 days, that is, the Mean Time Between Failures (MTBF) is 30 days. When it happens it takes 12 hours to reboot it, that is, Mean Time To Repair (MTTR) is 12 hours. The availability of server with these reliability data values is approximately.

1. 96.3 %
2. 97.3 %
3. 98.3%
4. 99.3%

C is Ans.

If we let A represent availability, then the simplest formula for availability is:

A = Uptime/(Uptime + Downtime)

MTBF is  Mean Time Between Failures

MTTR is Mean Time To Repair

A = MTBF / (MTBF+MTTR)

So MTBF=30day=30*24=720 hr

A=720/(720+12)

=98.3%