There are four machines and it is known that exactly two of them are faulty. They are tested, one by one, in a random order till both the faulty machines are identifies. Then the probability that only two tests are needed is:
1
\(\dfrac{1}{4}\)
2
\(\dfrac{1}{3}\)
3
\(\dfrac{1}{2}\)
4
\(\dfrac{1}{6}\)