The answer is approximately 99.89%. This composite SLA calculation for dependent Azure services is correct because when two components must both be available for the system to function, you multiply their individual availability percentages—0.999 (99.9%) times 0.9999 (99.99%) equals 0.9989, or 99.89%. On the Microsoft Azure Fundamentals AZ-900 exam, this concept tests your understanding of how composite SLAs differ from independent service SLAs; a common trap is to average the percentages or pick the higher number, but remember that dependencies always reduce overall availability. The key insight is that the chain is only as strong as its weakest link, and multiplication reveals the true combined uptime. For a quick memory tip, think “multiply when they’re tied together, add when they’re side by side”—dependent services multiply, while independent, redundant services would increase availability through a different formula.
AZ-900 Describe cloud concepts Practice Question
This AZ-900 practice question tests your understanding of describe cloud concepts. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company runs business-critical ERP software on Azure. If the ERP application has a 99.9% SLA and the Azure SQL Database backing it has a 99.99% SLA, what is the overall 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 99.89%
The composite SLA for dependent services is calculated by multiplying their individual SLAs. Here, 99.9% (0.999) multiplied by 99.99% (0.9999) equals approximately 0.9989, or 99.89%. This reflects the overall availability when both the ERP application and the Azure SQL Database must be operational for the system to function.
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.
✗
99.99% (take the highest SLA)
Why it's wrong here
You cannot take the highest individual SLA for dependent services; they must be multiplied.
✓
Approximately 99.89%
Why this is correct
0.999 × 0.9999 ≈ 0.9989, so the composite SLA is approximately 99.89%.
Related concept
Read the scenario before looking for a memorised answer.
✗
99.9% (take the lowest SLA)
Why it's wrong here
Taking the lowest SLA is incorrect; the composite SLA is lower than any individual SLA.
✗
200% because two services are running
Why it's wrong here
SLAs cannot exceed 100%; composite SLA is 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 mistakenly pick the lowest SLA (Option C) or highest SLA (Option A) instead of multiplying the probabilities, failing to recognize that composite SLA for dependent services is a product, not a min or max.
Detailed technical explanation
How to think about this question
Composite SLA calculation assumes independent failure probabilities; for n dependent services, the formula is ∏(SLA_i). In practice, Azure SLA credits are based on monthly uptime percentages, and a 99.9% SLA allows ~43 minutes of downtime per month, while 99.99% allows ~4 minutes. Real-world scenarios often include redundancy (e.g., active geo-replication) to raise the effective composite SLA above individual component SLAs.
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 99.89% — The composite SLA for dependent services is calculated by multiplying their individual SLAs. Here, 99.9% (0.999) multiplied by 99.99% (0.9999) equals approximately 0.9989, or 99.89%. This reflects the overall availability when both the ERP application and the Azure SQL Database must be operational for the system to function.
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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