Sources page biographical material

5.B.2. CROSSING A BRIDGE WITH A TORCH
New section.

Four people have to get across a bridge which is dark and needs to be lit with the torch. The torch can serve for at most two people and the gap is too wide to throw the torch across, so the torch has to be carried back and forth. The various people are of different ages and require 5, 10, 20, 25 minutes to cross and when two cross, they have to go at the speed of the slower. But the torch (= flashlight) battery will only last an hour. Can it be done? I heard this about 1997, when it was claimed to be used by Microsoft in interviewing candidates. I never found any history of it, until I recently found a discussion on Torsten Sillke's site: Crossing the bridge in an hour (www.mathematik.uni bielefeld.de/~sillke/PUZZLES/crossing-bridge), starting in Jun 1997 and last updated in Sep 2001. This cites the 1981 source and the other references below. Denote the problem with speeds a, b, c, d and total time t by

(a, b, c, d; t), etc. t is sometimes given, sometimes not.
Saul X. Levmore & Elizabeth Early Cook. Super Strategies for Puzzles and Games. Doubleday, 1981, p. 3 -- ??NYS. (5, 10, 20, 25; 60), as in the introduction to this section..

Heinrich Hemme. Das Problem des Zwölf-Elfs. Vandenhoeck & Ruprecht, 1998. Prob. 81: Die Flucht, pp. 40 & 105-106, citing a web posting by Gunther Lientschnig on 4 Dec 1996. (2, 4, 8, 10; t).

Dick Hess. Puzzles from Around the World. Apr 1997. Prob. 107: The Bridge.

(1, 2, 5, 10; 17). Poses versions with more people: (1, 3, 4, 6, 8, 9; 31) and, with a three-person bridge, (1, 2, 6, 7, 8, 19, 10; 25).

Quantum (May/Jun 1997) 13. Brainteaser B 205: Family planning. Problem (1, 3, 8, 10; 20).

Karen Lingel. Email of 17 Sep 1997 to rec.puzzles. Careful analysis, showing that the 'trick' solution is better than the 'direct' solution if and only if a + c > 2b. [Indeed, a + c - 2b is the time saved by the 'trick' solution.] She cites (2, 3, 5, 8; 19) and (2, 2, 3, 3; 11) to Sillke and (1, 3, 6, 8, 12; 30), from an undated website. Expressing the solution for more people seems to remain an open question.