HMMT 二月 2003 · 冲刺赛 · 第 42 题

HMMT February 2003 — Guts Round — Problem 42

[18] A tightrope walker stands in the center of a rope of length 32 meters. Every minute she walks forward one meter with probability 3 / 4 and backward one meter with probability 1 / 4. What is the probability that she reaches the end in front of her before the end behind her? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HARVARD-MIT MATHEMATICS TOURNAMENT, MARCH 15, 2003 — GUTS ROUND

解析

A tightrope walker stands in the center of a rope of length 32 meters. Every minute she walks forward one meter with probability 3 / 4 and backward one meter with probability 1 / 4. What is the probability that she reaches the end in front of her before the end behind her? 16 16 Solution: 3 / (3 + 1) After one minute, she is three times as likely to be one meter forward as one meter back. After two minutes she is either in the same place, two meters forward, or two 2 2 meters back. The chance of being two meters forward is clearly (3 / 4) and thus 3 = 9 2 times greater than the chance of being two back ((1 / 4) ). We can group the minutes into two-minute periods and ignore the periods of no net movement, so we can consider her to be moving 2 meters forward or backward each period, where forward movement 2 is 3 times as likely as backward movement. Repeating the argument inductively, we 16 eventually find that she is 3 times more likely to move 16 meters forward than 16 12 16 16 meters backward, and thus the probability is 3 / (3 + 1) that she will meet the front end of the rope first.