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6.Permutation and Combination
medium
A test consists of $6$ multiple choice questions, each having $4$ alternative ans wers 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
A
$135$
B
$140$
C
$125$
D
$130$
(JEE MAIN-2020)
Solution
$Ways ={ }^{6} C _{4} \cdot 1^{4} \cdot 3^{2} $
$=15 \times 9 $
$=135 $
Standard 11
Mathematics
