ITPEC IP October 2025 Question 14
The system configurations A through C are made up by connecting multiple identical devices. Which of the following is the list where A through C are sorted in descending order of availability? Here, ─▭─ represents a device. The parallel connection requires only one (1) of the devices to be functional, while the series connection requires all devices to be functional.
Explanation
C, B, A — parallel devices raise availability and devices in series lower it, so rank by how much each configuration depends on a single device.
Let each device have availability p, between 0 and 1:
- •A, two in series: p2
- •B, one device in series with a parallel pair: p × (1 − (1 − p)2)
- •C, three in parallel: 1 − (1 − p)3
With p = 0.9: A = 0.81, B = 0.9 × 0.99 = 0.891, C = 0.999. A and B share one series device, but B backs up its second device with a parallel partner, so B beats A; C stops only if all three fail.
Answer (d)
Why not others:
- •(a) — is the exact reverse, as if every added device lowered availability
- •(b) — puts C last, treating its parallel devices as if they were in series
- •(c) — ranks A above B, missing that B's parallel pair is more available than A's single second device
Key rule: Series: multiply the availabilities; parallel: 1 minus the product of the unavailabilities. Series lowers availability below any one device, parallel raises it above.
System Component · AI-written, may contain errors
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