ITPEC FE Morning April 2022 Question 16
Topic: System Component
Which of the following is the system configuration that has the highest availability? Here, when systems are connected in parallel, the systems are considered to be operational if at least one (1) of them is operating.
Explanation
Three 80% systems in parallel — a parallel group fails only when every member fails, so availability is 1 − (1 − a)n.
- •(a) single system: 0.99
- •(b) four at 70%: 1 − 0.34 = 1 − 0.0081 = 0.9919
- •(c) three at 80%: 1 − 0.23 = 1 − 0.008 = 0.992
- •(d) two at 90%: 1 − 0.12 = 1 − 0.01 = 0.99
The highest is 0.992.
Answer (c)
Why not others:
- •(a) — 0.99, below 0.992: one system's 1% downtime is more than the 0.8% chance that three 80% systems are all down
- •(b) — 0.9919, just below 0.992: 0.34 = 0.0081 is a larger failure probability than 0.23 = 0.008
- •(d) — 0.99, the same as the single 99% system
Key rule: Compare parallel configurations by the probability that all units are down at once, (1 − a)n; the smallest wins.
System Component · AI-written, may contain errors
This question comes from an official ITPEC past paper. ITPEC Practice is an independent study tool and is not affiliated with ITPEC. See the official FE past-paper collection or Report an issue.