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
Availability is MTBF divided by MTBF plus MTTR: 500/520, which equals 96.15%, not 97.50%. This figure is tempting because it resembles a plausible result from misapplying the formula, perhaps dividing 500 by 513 or rounding incorrectly, but it does not match the correct calculation.
- ✗
92.00%
Why it's wrong here
Availability equals MTBF divided by MTBF plus MTTR, which is 500/520 or 96.15%, not 92.00%. This value is tempting as a rough estimate, but it corresponds to an MTTR of roughly 43 hours, not the 20 hours stated in the scenario.
- ✓
96.15%
Why this is correct
Availability equals MTBF divided by total time (MTBF plus MTTR): 500 ÷ 520 = 0.9615, or 96.15%. This satisfies the stem's requirement to derive availability from the given 500-hour MTBF and 20-hour MTTR, correctly treating repair time as the only downtime component.
- ✗
95.00%
Why it's wrong here
Availability is 500 divided by 520, giving 96.15%, so 95.00% is incorrect. The figure is tempting because it is a round number close to the true result, but it would only arise from a different MTBF or MTTR pairing, not the values given in the stem.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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