There are four machines and it is known that eactly two of them are faulty. They are tested, one by one, in a random order till both the faulty machines are identified. The probability that only two tests are needed is

Options

A. `(1)/(3)`
B. `(1)/(6)`
C. `(1)/(2)`
D. `(1)/(4)`

Answer

B

Explanation

Probability that only two tests are needed =Probability that the first machine tested is faulty `xx` Probability that the second machine tested is faulty `=(2)/(4)xx(1)/(3)=(1)/(6)`