A test consists of 6 multiple choice questions, each haying 4 alternative answers 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

Number of ways to select four questions from six questions =6C4And number of ways to answer these questions correctly=1x1x1x1=1And number of ways to answer remain two questions wrongly = 3 x 3 = 9Required number of ways=6C4×1×9=6!2!4!×9=6×52×9=135