There are four machines and it is known that exactly two of them are faulty. They are tested, one by one, is a random order till both the faulty machines are identified. Then the probability that only two tests are needed
Options:
A .  
13
B .  
16
C .  
12
D .  
14
Answer: Option B : B This is a problem of without replacement. P=onedef.from2def.anyonefrom4×1def.fromremaining1def.anyonefromremaining3 Hence required probability = 24×13=16 Aliter : Number of ways in which two faulty machines may be detected (depending upon the test done to identify the faulty machines) = 4C2=6 Number of favourable cases = 1 [When faulty machines are identified in the first and the second test]. Hence required probability = 16
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