Availability of a system is roughly the:

Question

Options

• AProduct of the individual subsystem availabilities.
• BSum of the individual system availabilities.
• CThe difference of the individual subsystem.
• DThe quotient of the individual subsystem availability.

• A
  73% (29)
• B
  5% (2)
• C
  13% (5)
• D
  10% (4)

Explanation

When subsystems are arranged in series (each must work for the whole system to work), overall system availability is found by multiplying the individual availability values together - for example, two subsystems each at 0.9 availability yield a combined availability of 0.9 × 0.9 = 0.81. This reflects that each component's chance of being up compounds the overall probability.

Why the distractors are wrong:

• B (Sum): Adding availabilities would produce values greater than 1.0, which is impossible for a probability - this only applies to certain redundant (parallel) configurations, and even then it's not a simple sum.
• C (Difference): Subtracting availabilities has no meaningful interpretation in reliability engineering.
• D (Quotient): Dividing one availability by another is also meaningless in this context and could produce values greater than 1.

Memory tip: Think of availability like rolling dice - to get all dice to land on a specific face simultaneously, you multiply the individual probabilities. Systems in series work the same way: every component must be "up" at the same time, so you multiply.

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