- A
97.50%
Why wrong: Incorrect.
- B
92.00%
Why wrong: Incorrect.
- C
96.15%
Correct calculation.
- D
95.00%
Why wrong: Incorrect calculation.
Question 838 of 984
CISA Practice Question: Information Systems Operations and Business Resilience
This CISA practice question tests your understanding of information systems operations and business resilience. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A system has a Mean Time Between Failures (MTBF) of 500 hours and a Mean Time To Repair (MTTR) of 20 hours. What is the availability of the system?
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
✓
96.15%
Availability is calculated as MTBF / (MTBF + MTTR). With MTBF = 500 hours and MTTR = 20 hours, availability = 500 / (500 + 20) = 500 / 520 ≈ 0.9615, or 96.15%. This formula measures the proportion of time the system is operational, directly reflecting its resilience and recoverability.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
97.50%
Why it's wrong here
Incorrect.
- ✗
92.00%
Why it's wrong here
Incorrect.
- ✓
96.15%
Why this is correct
Correct calculation.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
95.00%
Why it's wrong here
Incorrect calculation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may incorrectly compute availability as (MTBF - MTTR)/MTBF or simply subtract MTTR/MTBF from 1 without using the correct denominator, leading to plausible but wrong percentages like 96% or 95%.
Detailed technical explanation
How to think about this question
Availability in this context is often called 'inherent availability' (Ai), assuming no preventive maintenance downtime. The formula MTBF/(MTBF+MTTR) derives from the steady-state probability of a repairable system being up, modeled as a continuous-time Markov chain. In real-world IT operations, MTBF and MTTR are estimated from historical failure and repair logs, and availability calculations directly impact SLA commitments and redundancy planning.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the CISA exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Information Systems Operations and Business Resilience — study guide chapter
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FAQ
Questions learners often ask
What does this CISA question test?
Information Systems Operations and Business Resilience — This question tests Information Systems Operations and Business Resilience — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 96.15% — Availability is calculated as MTBF / (MTBF + MTTR). With MTBF = 500 hours and MTTR = 20 hours, availability = 500 / (500 + 20) = 500 / 520 ≈ 0.9615, or 96.15%. This formula measures the proportion of time the system is operational, directly reflecting its resilience and recoverability.
What should I do if I get this CISA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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.
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