@mailashish89 said: yaar koi probability wala question samjhao, Ques no.8 20 Questions, each having 5 answer choices, etc.
Bayes theorem (Conditional Probability) ka application tha. If you know it, it is a basic question. Stating it here, since you probably are new to it:
If A and B are two events, then P(A|B) denotes the "conditional probability of A, given B" ie the probability of A occurring if B has occurred.
In general, P(A|B) x P(B) = P(A or B) = P(B|A) x P(A).
From the above, P(B|A) = P(A|B) x P(B) / P(A).
Now, for the question, take A as the event of "marking the correct answer", B be the event of "not knowing the answer", and B' as the opposite event of B ie "knowing the answer".
We have to find P(B|A).
Now P(A|B) = P ( Marking correct answer given the answer is not known) = 1/5 (since it is a random guess from 5 options)
P(B) = Prob of not knowing the answer = 1/5
P(A) = Prob of marking correct answer = P(A|B) x P(A) + P(A|B') x P(B') = 1/5 * 1/5 + 1 * 4/5 = 21/25
Therefore P(B|A) = (1/25)/(21/25) = 1/21