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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 :

1. 6 5 × (15)!
2. 5 6 × 15
3. (15)! × 6 !
4. 5! × 6!

Correct answer: (15)! × 6 !

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