There are four machine and it is know that exactly two of them are faulty, They are tested. One by one in a random order till both the faulty machines are identified. Then the probability that only two tests are needed is?

Correct Answer: 1/6

Explanation:

First we choose 1 machine out of the given 4 , the probability that it is faulty is 2/4 (favorable outcomes/possible outcomes)
Now we have to pick the second faulty machine the probability of doing so is 1/3.
Hence overall probability is 2/4 x 1/3 = 1/6

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