CISA Practice Question: Information Systems Operations and Business Resilience
An IS auditor is reviewing the availability management process. The auditor calculates that the mean time between failures (MTBF) is 200 hours and the mean time to repair (MTTR) is 20 hours. What is the availability percentage?
⚠ Common exam trap
CISA often tests the availability formula by presenting MTBF and MTTR values that produce a non-obvious percentage — the trap is inverting the ratio (MTTR/MTBF) or forgetting to add MTTR to the denominator.
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
✓
90.91%
Availability is calculated as MTBF / (MTBF + MTTR). With MTBF = 200 hours and MTTR = 20 hours, availability = 200 / (200 + 20) = 200 / 220 = 0.90909..., which is 90.91%. This formula expresses the proportion of time the system is operational relative to total time (uptime + downtime).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
90.91%
Why this is correct
Availability equals MTBF divided by the sum of MTBF and MTTR: 200 / (200 + 20) = 200 / 220 = 0.9091, giving 90.91%. The 20-hour repair window is downtime, so it must be added to the 200-hour uptime interval in the denominator.
- ✗
99.00%
Why it's wrong here
Availability is MTBF divided by MTBF plus MTTR: 200/220, giving 90.91%, not 99%. The 99% figure tempts auditors who assume near-perfect uptime, but it would require an MTTR of roughly two hours against this 200-hour MTBF, not twenty.
- ✗
95.00%
Why it's wrong here
Availability is MTBF divided by MTBF plus MTTR: 200/220, which equals 90.91%, not 95%. The figure 95% is tempting because it mirrors the common 5% downtime assumption, but that derives from a 19:1 ratio, whereas this scenario's ratio is 10:1.
- ✗
80.00%
Why it's wrong here
80.00% results from dividing MTTR by MTBF (20 ÷ 200), but availability is MTBF divided by total time (MTBF + MTTR), giving 90.91%. The figure tempts because MTTR genuinely drives downtime, so it feels central; it would be the right calculation when expressing the proportion of time spent repairing.
Go deeper
Related to this question
About these practice questions
One of 934 original CISA practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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
This CISA practice question is part of Courseiva's free ISACA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISA exam.