The number of functions f from {1, 2, 3, ...., 20} onto {1, 2, 3, ...., 20} such that f(k) is a multiple of 3,
whenever k is a multiple of 4, is :
- A65 $$ \times $$ (15)!
- B56 $$ \times $$ 15
- C(15)! $$ \times $$ 6!
- D5! $$ \times $$ 6!
✓ Correct answer: C
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