HMMT 十一月 2008 · 冲刺赛 · 第 20 题

HMMT November 2008 — Guts Round — Problem 20

[ 11 ] You have a die with faces labelled 1 through 6. On each face, you draw an arrow to an adjacent face, such that if you start on a face and follow the arrows, after 6 steps you will have passed through every face once and will be back on your starting face. How many ways are there to draw the arrows so that this is true?

解析

[ 11 ] You have a die with faces labelled 1 through 6. On each face, you draw an arrow to an adjacent face, such that if you start on a face and follow the arrows, after 6 steps you will have passed through every face once and will be back on your starting face. How many ways are there to draw the arrows so that this is true? Answer: 32 There are 4 choices for where to go from face 1. Consider the 4 faces adjacent to 1. We can visit either 1, 2, or 3 of them before visiting the face opposite 1. If we only visit one of these adjacent faces, we have 4 choices for which one, then we visit face 6, opposite face 1, then we visit the remaining 3 faces in one of two orders - for a total of 8 ways. If we visit 2 adjacent faces first, there is 8 choices for these two faces, then 2 choices for the path back from face 6 to face 1. Lastly, there are 8 ways to visit three of the adjacent faces before visiting the opposite face. These choices give 32 total.