The Mensa Correctional Institute. Issue 4 (21st April 2018)

• Record Type: Journal Article
• Authors: Carter, Larry; Wagon, Stan
• Is Part Of: American Mathematical Monthly. Volume 125:Issue 4(2018)
• Page Start: 306
• Page End: 319
• Publication Date: 2018-04-21
• Subjects: Primary 91A28 -- Secondary 05A15; 05D99; Mathematics -- Periodicals; 510.5
• DOI: 10.1080/00029890.2018.1418100
• Languages: English
• ISSNs: 0002-9890

Abstract

Abstract: We investigate the following puzzle and several variations, some of which have quite surprising answers. Alice and Bob are in prison under the care of warden Charlie. Alice will be brought into Charlie's office and shown 52 cards, face-up in a row in an arbitrary order. Alice can interchange two cards. Charlie then turns all cards face-down in their places and Alice leaves the room. Then Bob is brought in and Charlie calls out a random target card. Bob can turn over cards, one after another, at most 26 times as he searches for the target. Both prisoners are freed if Bob finds the target. Find a strategy that never fails.