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Q. An instrument consists of two units. Each unit must function for the instrument to operate. The reliability of the first unit is 0.9 & that of the second unit is 0.8. The instrument is tested & fails. The probability that "only the first unit failed & the second unit is sound" is :

Probability - Part 2

Solution:

A: the instrument has failed
$B_1$ : first unit fails and second is healthy
$B _2$ : first unit healthy and second unit fails
$B _3$ : both fails
$B _4$ : both healthy

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