JEE Advanced 2020 Mathematics question
An engineer is required to visit a factory for exactly four days during the first 15 days of every month and it is mandatory that **no** two visits take place on consecutive days. Then the number of all possible ways in which such visits to the factory can be made by the engineer during 1-15 June 2021 is
Answer: 495
Solution
We must choose 4 days out of 15 with no two consecutive. Mark the 11 non-visiting days in a row; they create 12 gaps (including the two ends),
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