JEE Main 2019 Mathematics question
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:
Answer: (C)
Solution
4, 8, 12, 16, 20 map injectively into the six multiples of 3: 6!/1! ways; the other 15 values can be arranged in 15! ways.