The answer is approximately 99.85%. This is correct because when two services both must be available, you multiply their individual SLAs to find the composite SLA, reflecting that the overall availability decreases as dependencies increase. Service A at 99.9% (0.999) times Service B at 99.95% (0.9995) equals 0.9985005, which rounds to 99.85%. On the Microsoft Azure Fundamentals AZ-900 exam, this tests your understanding of composite SLA calculation for Azure multiple services in an “and” scenario, where a common trap is mistakenly averaging the SLAs or adding them instead of multiplying. Remember that requiring both services introduces more potential failure points, so the composite is always lower than the lowest individual SLA. A handy memory tip: “And means multiply, so the number gets smaller.”
AZ-900 Describe cloud concepts Practice Question
This AZ-900 practice question tests your understanding of describe cloud concepts. 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.
When using Azure, a company wants to calculate the composite SLA for two services used together where both must be available. Service A has a 99.9% SLA and Service B has a 99.95% SLA. What is the composite SLA?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Approximately 99.85%
When two services must both be available (an 'and' scenario), the composite SLA is calculated by multiplying their individual SLAs. Service A at 99.9% (0.999) and Service B at 99.95% (0.9995) gives 0.999 × 0.9995 = 0.9985005, which rounds to approximately 99.85%. This reflects the increased probability of failure when multiple dependencies are required.
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.
✗
99.95% (the higher of the two SLAs)
Why it's wrong here
The composite SLA is not the higher individual SLA — dependent services multiply their SLAs.
✗
99.9% (the lower of the two SLAs)
Why it's wrong here
The composite SLA is not the lower individual SLA either — it is calculated by multiplication.
✓
Approximately 99.85%
Why this is correct
Composite SLA = 0.999 × 0.9995 ≈ 0.99850 or ~99.85%, lower than either individual SLA.
Related concept
Read the scenario before looking for a memorised answer.
✗
100% because Azure guarantees maximum availability
Why it's wrong here
No service guarantees 100% SLA; composite SLAs are always lower when services depend on each other.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the 'and' (multiplicative) scenario with the 'or' (redundancy) scenario, mistakenly picking the higher or lower SLA instead of performing the multiplication.
Detailed technical explanation
How to think about this question
The composite SLA calculation uses the formula SLA_total = SLA_A × SLA_B for dependent services, derived from probability theory for independent events. In real-world scenarios, this matters for multi-tier applications (e.g., web frontend + database), where each additional component reduces overall availability. For example, adding a third service with 99.9% SLA would drop the composite to approximately 99.75%, highlighting why architects design for redundancy or choose higher SLAs for critical paths.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-900 question in full detail.
Describe cloud concepts — This question tests Describe cloud concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Approximately 99.85% — When two services must both be available (an 'and' scenario), the composite SLA is calculated by multiplying their individual SLAs. Service A at 99.9% (0.999) and Service B at 99.95% (0.9995) gives 0.999 × 0.9995 = 0.9985005, which rounds to approximately 99.85%. This reflects the increased probability of failure when multiple dependencies are required.
What should I do if I get this AZ-900 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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