IPA IP 2025 Question 94
A question IPA Japan published in 令和7年度 ITパスポート試験 公開問題 問94, translated into English. ITPEC is based on the same exams.
A parallel system is to be configured with devices whose availability is 0.9. At least how many devices are required to configure the parallel system so that the availability of this system is 0.999? Here, when the parallel system is configured, the system is considered to be operating normally if at least one (1) device is operating.
Explanation
3 devices — a parallel system fails only when every device is down at once.
Each device is down with probability 1 − 0.9 = 0.1. For n devices in parallel, the system is down only when all n are down, so its availability is 1 − 0.1n:
- •n = 2: 1 − 0.01 = 0.99, short of 0.999
- •n = 3: 1 − 0.001 = 0.999, the target
Three is the smallest number that reaches 0.999.
Answer (b)
Why not others:
- •(a) 2 — gives only 0.99
- •(c) 4 and (d) 5 — reach 0.9999 and 0.99999, more than needed; the question asks for the minimum
Key rule: Parallel availability = 1 − (1 − A)n: multiply the devices' unavailabilities, not their availabilities (that is the series formula).
System Component · AI-written, may contain errors
This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.