A request passes through two components in a row, each 99.9% available. What is the availability of the whole path?

Options

• ✓ About 99.8% Correct
• ✗ 99.9%
• ✗ About 99.9999%
• ✗ 99.95%

Why

In series, both must be up: 0.999 × 0.999 ≈ 0.998. Every hop lowers the total. Redundancy works the other way: two copies in parallel are down only when both are, 1 − 0.001² = 99.9999%.

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