Question

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

• A. $\frac{1}{3}$
• B. $\frac{1}{6}$
• C. $\frac{1}{2}$
• D. $\frac{1}{4}$

Answer 1

Answer: (B)

Probability that only two tests are needed = Probability that the first machine tested is faulty $\times $ Probability that the second machine tested is faulty
=$\frac{2}{4} \times \frac{1}{3}=\frac{1}{6}$