JEE Main 2022 Mathematics question
Let be a 4-element permutation with {1, 2, 3, ….., 100} for and for , such that either , , are consecutive integers or , , are consecutive integers. Then the number of such permutations is equal to .
Answer: 18915
Solution
There are 98 sets of three consecutive integers and 97 sets of four consecutive integers. The number of arrangements with , , consecutive (in order) is , since is any of the remaining 97 numbers. The same
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