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 faulty machines are identified.Then the probability that only two tests are needed,is
Answer is 1/12......
No pet its 1/12......
Let W and T denote Working machine.
Let F and G denote Faulty machine.
P(F) at first go=1/4 .......
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You have two faulty machines.
so n(A) = 2
now you have 2 machines out of four machines so you can select them in 4C2 ways
so n(S) = 12
so P(A) = 2/12
If you consider both machines to be identically faulty the you are correct.....
But if the are not identical mecanically faulty the i am correct
Such simple wont come in IIT-JEE