CISA Practice Question: Information Systems Operations and Business Resilience
A system has a Mean Time Between Failures (MTBF) of 200 hours and a Mean Time To Repair (MTTR) of 20 hours. What is the availability of the system?
⚠ Common exam trap
CISA often tests the availability formula, and candidates frequently confuse MTBF and MTTR or incorrectly use MTTR/MTBF instead of MTBF/(MTBF+MTTR).
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
90.9%
Availability is calculated as MTBF / (MTBF + MTTR). With MTBF = 200 hours and MTTR = 20 hours, availability = 200 / (200 + 20) = 200/220 = 0.90909..., which rounds to 90.9%. This formula reflects the proportion of total time the system is operational versus the total time including repair.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
95.0%
Why it's wrong here
Availability of 95.0% corresponds to an MTTR of about 10.5 hours against a 200-hour MTBF, not the stated 20 hours. The value tempts as a plausible high-availability target, but it ignores the supplied repair time.
- ✗
91.0%
Why it's wrong here
Close but not exact.
- ✗
90.0%
Why it's wrong here
90.0% misreads the ratio as MTTR divided by MTBF, but availability is MTBF ÷ (MTBF + MTTR), giving 200 ÷ 220 ≈ 90.9%. The figure is tempting because 20 hours of downtime against 200 hours of uptime looks like a clean ten per cent loss, yet the repair time belongs in the denominator alongside the failure interval.
- ✓
90.9%
Why this is correct
Availability equals MTBF divided by the sum of MTBF and MTTR: 200 / (200 + 20) = 200/220 = 0.909, giving 90.9%. The 20-hour repair window is the only downtime component, so the system is available for 200 of every 220 hours.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official ISACA exam blueprint
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