ITPEC FE Subject A October 2024 Question 12

System Availability (series-parallel) — calculate combined availability of a mixed topology.

Given devices: A(0.8), B(0.7), C(0.9), D(0.7), E(0.8)

Step 1 — Parallel groups:
- A ∥ B = 1 - (1-0.8)(1-0.7) = 1 - 0.06 = 0.94
- D ∥ E = 1 - (1-0.7)(1-0.8) = 1 - 0.06 = 0.94

Step 2 — Series connection:
- Total = 0.94 × 0.9 × 0.94 = 0.795

Why not others:
- (a) 0.003 — multiplied all five availabilities in series
- (b) 0.031 — incorrect formula application
- (c) 0.282 — likely swapped parallel/series formulas

Key rules:
- Parallel: 1 - (1-A)(1-B) — system works if either device works
- Series: A × B — system works only if both devices work

AI-generated — may contain errors