The climbing ant爬墙的小蚂蚁
An ant climbs a 10 m wall. Each day it climbs up 3 m, but each night it slides back down 2 m. On which day does it first reach the top?
一只小蚂蚁爬一面 10 米高的墙。它每天白天向上爬 3 米,晚上又滑下 2 米。它在第几天第一次爬到墙顶?
Hint提示
Careful — once it reaches the top during the day, it doesn't slide back.注意:白天一旦爬到顶,就不会再滑下来了。
Answer & reasoning答案与思路
Day 8. After each full day and night it gains 1 m, so after 7 days and nights it is at 7 m. On day 8 it climbs 3 m and reaches 10 m before night comes. (Many students answer 10 — that's exactly the trap!)第 8 天。每过一整天(白天 + 晚上)净上升 1 米,7 天 7 夜后在 7 米处;第 8 天白天再爬 3 米,就到达 10 米,不会再滑下。很多同学会答 10 天——这正是陷阱所在!




