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Two independent displays each fail at \( 10^{-4} \) per hour. The probability both fail in the same hour, assuming independence, is

  • A.\( 10^{-4} \)
  • B.\( 2 \times 10^{-4} \)
  • C.\( 10^{-2} \)
  • D.\( 10^{-8} \)

Answer: D

Correct answer: \( 10^{-8} \)

For independent events the joint probability is the product:

\[ 10^{-4} \times 10^{-4} = 10^{-8} \]

The result is only as good as the independence assumption, which is the real lesson. Two displays sharing a symbol generator, a power supply, a cooling duct or a software load are not independent, and a common cause can defeat the calculation entirely. This is why redundancy is pursued through dissimilarity and segregation, not merely by fitting two of something.

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