CISA Practice Question: Information Systems Operations and Business Resilience
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?
⚠ Common exam trap
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%.
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.
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.
- ✗
95.00%
Why it's wrong here
Incorrect calculation.
Go deeper
Related to this question
About these practice questions
One of 995 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 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.