By: DUAN Yanlin, Alan (Jockey Club Academy Hall)
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
文: 段延麟 (賽馬會群智堂)