[Puzzle Solution] 【謎題:你能幫這十個學生拿到A+嘛?——解答篇】

puzzle-solved

[Puzzle Solution]

By: DUAN Yanlin, Alan (Jockey Club Academy Hall)

Image: itembrowser.com

Hi! Here comes the solution for the previous puzzle at https://goo.gl/Y5P4an:

Before the start of the course, 10 students choose one as their ‘leader’. Each time, whenever a person (non-leader) goes into the room, if the coin is with its head up, he/she flips it, and will only do this once (so not touch the coin if entering the room in later times); if the coin is with its tail up, he/she does nothing.

As for the leader, he/she will only flip the coin back to its head up when he/she sees its tail is up, otherwise leader will do nothing. When the leader flips the coin for 9 times, he/she can then claim that all 10 students have been to this room by now.

Do you understand why this strategy would work? Just think the coin as a ‘signal emitter’. All other students use ‘tail up’ to send a signal to the leader, saying that ‘hey, someone has been to the room!’ But they will only emit this signal once, and no one except the leader will receive this signal and reset the coin. Although this process may take a while, it will definitely ensure that every student can get an A+ at the end of semester!

Did you get it right?

Puzzle adapted from: http://www.cut-the-knot.org

【謎題:你能幫這十個學生拿到A+嘛?——解答篇】

文: 段延麟 (賽馬會群智堂)
圖: itembrowser.com

哈嘍!這裡就是之前謎題的答案啦 https://goo.gl/Y5P4an:

在課程正式開始之前,十個學生將選其中一名作為領隊。每一次如果不是領隊的學生進入到密室中,並且發現硬幣正面朝上,他/她就會把硬幣翻轉——並且他/她只會做這麼一次(所以如果之後進入到密室中,他/她就不會再動硬幣);然而如果發現硬幣反面朝上,他/她則什麼都不做。

如果是領隊進入到房間並且發現硬幣反面朝上,領隊就會把硬幣反轉,否則就什麼都不做。當領隊將硬幣反轉9次後,他就可以選擇告訴老師:所有十位同學都已經進入過這個房間啦!

為什麼這個策略會成功呢?把這個硬幣想象成一枚信號發射器吧:所有非領隊學生都用「反面朝上」來向領隊釋放一個信號,就好像在說「嘿,之前有人進來過啦!」。但每個人只會釋放信號一次,而且除了領隊之外的人都不會去接受這個信號並重置硬幣。雖然這個過程可能需要點時間,但是一定可以保證所有人都在學期末拿到A+哦!

你答對了麼?

謎題改編自:http://www.cut-the-knot.org

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