A system has an MTBF (mean time between failures) of 500 hours, and the availability target in its SLA is at least 99.5%. Assuming the MTBF does not change and calculating availability = MTBF ÷ (MTBF + MTTR), which of the following is closest to the maximum MTTR (mean time to repair) that meets the target?

Answer: B. 2.5 hours

Solving 500 ÷ (500 + MTTR) ≥ 0.995 gives MTTR ≤ 500 × 0.005 ÷ 0.995 ≈ 2.51 hours, so the maximum is about 2.5 hours. With 1.5 hours, the availability is 500 ÷ 501.5 ≈ 0.997, which meets the target, but it is not the maximum. 3.6 hours is the total outage time allowed in a 720-hour month (720 × 0.005), not the repair time per failure. Monitoring that detects failures quickly, spare parts on hand, well-prepared procedures and automatic switchover are effective in shortening the MTTR.

• A. This meets the target but is not the largest value allowed.
• B. Correct. MTTR ≤ 500 × 0.005 ÷ 0.995 ≈ 2.51 hours.
• C. This is the total outage time allowed in a month (720 hours), mistaken for the repair time per failure. With an MTTR of 3.6 hours, the availability would be about 0.993.