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. 1/3
• B. 1/6
• C. ½
• D. ¼

Answer 1

Answer: (B)

1/6

Answer 2

The probability that only two tests are needed =

(probability that the first machine tested is faulty) x (probability that the second machine tests is faulty given the first machine tested is faulty) = $\frac { 2 } { 4 } \times \frac { 1 } { 3 } = \frac { 1 } { 6 }$ .