Four Men, A Bridge, And A Lantern Riddle
Published: Nov 30, 1999
Riddle: Four men are trying to cross a small bridge. They can each individually cross it in 1, 2, 5, and 10 minutes respectively. To cross, they must carry a lantern to light the way, and at most two can cross at the same time (when two people cross it takes the longer time for both of them). Can they all get across in 17 minutes? (Remember they have to bring the lantern back for the other people to use!)
Answer: Yes, it is possible in 17 minutes. Here is how it works:
1 2 5 10 ---- _ _ _ __ total = 0
_ _ 5 10 ---- 1 2 _ __ total = 2
1 _ 5 10 ---- _ 2 _ __ total = 3
1 _ _ __ ---- _ 2 5 10 total = 13
1 2 _ __ ---- _ _ 5 10 total = 15
_ _ _ __ ---- 1 2 5 10 total = 17.
F
1 2 5 10 ---- _ _ _ __ total = 0
_ _ 5 10 ---- 1 2 _ __ total = 2
1 _ 5 10 ---- _ 2 _ __ total = 3
1 _ _ __ ---- _ 2 5 10 total = 13
1 2 _ __ ---- _ _ 5 10 total = 15
_ _ _ __ ---- 1 2 5 10 total = 17.
F
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