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燃烧绳子计时 45 分钟

Burning Ropes

专题
Brainteaser / 脑筋急转弯
难度
L2

题目详情

有两根绳子，每根从任意一端点燃都会在恰好 1 小时内烧完，但燃烧速度不均匀。

你可以从任意端点点燃。

如何精确计时 45 分钟？

You have two ropes. Each rope takes 1 hour to burn completely, but they burn unevenly along their length. You need to measure 45 minutes. You can light the ropes from any end(s). How do you do it?

解析
  1. 同时点燃：绳子 1 的两端、绳子 2 的一端。

绳子 1 会在 30 分钟烧完。

  1. 当绳子 1 烧完时，绳子 2 还剩“30 分钟的燃烧量”。立即点燃绳子 2 的另一端，它会在 15 分钟烧完。

总计 30+15=4530+15=45 分钟。

Original Explanation

  1. Light Rope 1 from both ends and Rope 2 from one end simultaneously.
    • Rope 1 will finish burning in 30 minutes (because lighting from both ends halves the burn time), so at that moment, Rope 2 has 30 minutes left.
  2. Immediately light the other end of Rope 2. This will burn the remaining 30-minute length in 15 minutes.
  3. Total = 30 + 15 = 45 minutes.