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IT Service Management and Systems Audit

During 30 days (720 hours) of operation, a system failed 3 times, and the total outage time due to the failures was 6 hours. Which of the following is the MTBF (mean time between failures) calculated from the results for this period?

  1. A238 hours
  2. B240 hours
  3. C714 hours
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Answer: A. 238 hours

MTBF is the total operating time divided by the number of failures. The operating time is 720 − 6 = 714 hours, so MTBF is 714 ÷ 3 = 238 hours. MTTR (mean time to repair) is 6 ÷ 3 = 2 hours, and the availability is 238 ÷ (238 + 2) ≈ 0.992, which equals 714 ÷ 720. 240 hours divides 720 hours by 3 without subtracting the outage time, and 714 hours is not divided by the number of failures.

  • A. Correct. (720 − 6) ÷ 3 = 238 hours.
  • B. This divides 720 hours by 3 without subtracting the 6 hours of outage.
  • C. This is the total operating time, not divided by the number of failures.
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