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 identified. Then the probability that only two tests are needed is:

1/3 1/6 1/2 1/4

1/6

P(required) = $\frac{select\, '2'\, faulty\, machines}{select\, '2'\, machines\, out\, of\, '4'\, machines}=\frac{{^2C}_2}{{^4C}_2}=\frac{1}{6}$