Four people on a bridge. (Posted on 2002-04-14)

	Submitted by levik
Rating: 3.6875 (16 votes)
Solution:	(Hide)
The shortest time is 17 minutes. The key to this problem is that to minimize the time spent, we need to have persons C and D to cross together. However, to avoid one of them having to make the trip back, we need to ensure that when they cross, someone is waiting for them on the other side to take the flashlight back. So we send A and B to the other side first. (This takes 2 minutes.) Then one of them returns with the flashlight (it doesn't matter who - the result will come out the same, but let's assume A comes back - making the total elapsed time 3 minutes.) Now C and D cross together, taking 10 minutes (and bringing our total to 13). B then takes the flashlight and returns to help A get across - taking 2 minutes to go each way, and yielding the result of 17 minutes. (You can verify that the result is the same if it is B and not A who returns with the flashlight initially.)

	Subject	Author	Date
	Puzzle Solution	K Sengupta	2022-07-16 00:50:02
	my solution/your solution	Awdreye	2005-06-28 08:07:19
	WHAT KIND OF SOLDIER ARE YOU, RECRUIT?	john	2005-06-16 19:47:56
	re: without to much cheking...	john	2005-06-16 19:30:18
	re(2): Repeat	Ken Haley	2004-11-04 05:35:36
	re: Repeat	Jim	2004-07-20 12:06:44
	without to much cheking...	Cesar Ali	2004-07-08 10:43:54
	simple answer	omar	2004-02-21 15:37:57
	re: Not that long - Too slow	Brian Smith	2003-10-15 09:44:25
	Not that long	Krysta Harty	2003-10-14 19:53:32
	ummmm	Lewis	2003-06-07 08:04:18
	Lowest solution	Danny	2003-02-06 06:36:51
	Repeat	cges	2002-12-12 06:08:46
	No Subject	Amanda	2002-10-08 10:03:43
	4people/bridge	baconfats	2002-05-11 03:32:32