HMMT 二月 2000 · POW 赛 · 第 4 题
HMMT February 2000 — POW Round — Problem 4
题目详情
- Find the n um b er of rearrangmen ts of 12345 (in luding 12345) su h that none of the follo wing is true: 1 is in p osition 5, 2 is in p osition 1, 3 is in p osition 2, 4 is in p osition 4, and 5 is in p osition 3.
解析
- This is just derangemen ts of 23541. D = 44 . 5 n