CISA Practice Question: Information Systems Operations and Business Resilience
A company's availability monitoring shows that a critical application has an average MTBF of 720 hours and an average MTTR of 4 hours. What is the availability percentage?
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
✓
99.45%
Availability = MTBF / (MTBF + MTTR) = 720 / (720 + 4) = 720 / 724 ≈ 0.994475, or 99.45%.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
99.72%
Why it's wrong here
Availability is MTBF divided by MTBF plus MTTR: 720/724 = 99.448%, so 99.72% is arithmetically wrong. It is tempting because it resembles a plausible uptime figure, but it corresponds to roughly two hours of downtime per 720-hour cycle rather than the stated four.
- ✗
99.17%
Why it's wrong here
99.17% corresponds to MTTR of about six hours (720/726), not four. It is tempting because it is close to the correct 99.448% and looks like a rounding variant, but substituting the stem's actual MTTR of four hours gives 720/724 = 99.448%.
- ✗
99.95%
Why it's wrong here
720/724 yields 99.448%, not 99.95%; that value would require an MTTR near 0.36 hours. It is tempting because 99.95% is a common five-nines-adjacent service target, but the stem's four-hour MTTR fixes the result at 99.448%.
- ✓
99.45%
Why this is correct
Availability equals MTBF divided by the sum of MTBF and MTTR: 720 ÷ (720 + 4) = 720 ÷ 724 = 0.99447, which rounds to 99.45%. This satisfies the stem's requirement to convert the given reliability and repair figures into an availability percentage.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.