Ten people on a war games weekend come to a railroad bridge. It is midnight and there is no moon. Crossing is very dangerous because the ties are slippery and unevenly spaced.
The people are of differing ages and fitness. The times it takes each person to cross individually are 20, 21, 23, 27, 29, 31, 34, 37, 41, and 43 minutes respectively. Each person knows everyone's time.
They have just 2 small flashlights, and each one casts only enough light to allow two people to cross safely. Whenever 2 people cross together, they cross in the time of the slower person.
The flashlights can be handed off at either end, but not part way. Crossers never turn around or stop on the bridge.
Everyone can get across in 3 hours (180 minutes) or less. How is this accomplished?
(In reply to
Solution by Lewis)
If their seargent is interested in the team getting across the bridge as fast as possible, a quicker solution puts more work on the fittest member:
Send 20, 37, 41 and 43 across with both lights. Send 20 back with both lights for a total time of 63 minutes.
Send 20, 29, 31 and 34 across with both lights. Send 20 back with both lights for a total time of 117 minutes.
Now just send 20, 21, 23 and 27 across with both lights. Everybody is across in only 144 minutes.
re: Solution
