Question:
A test consists of 6 multiple choice questions, each having 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
Solution:
Ways $={ }^{6} \mathrm{C}_{4} \cdot 1^{4} \cdot 3^{2}$
$=15 \times 9$
$=135$
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