A test consists of 6 multiple choice questions, each having 4 alternative answer of which only one is correct. The number of ways, in which a candidate answers all six questions such that exactly four of the answers are correct, is _____

 $$\begin{gathered} we can select 4 questions out of 6 questions in C(6,4) ways and select the correct \\ options in only 1 way.Now we can select wrong options for the remainng \\ 2 questions in 3\times 3 ways. \end{gathered}$$

$$\begin{gathered} number of ways =C(6,4)\times 1\times 3\times 3 \\ =135 \end{gathered}$$