In a test an examiner either guesses or copies or knows the answer to a multiple choice question with $4$ choices. The probability that he/she makes a guess is $\frac{1}{3}$. The probability that he/she copies the answer is $\frac{1}{6}$. If the probability that the answer is correct, given that he/she copied it is $\frac{1}{8}$, then the probability that he/she knows the answer to a question given that he/she correctly answered it, is