this was set as the 1st stage in an interview program for a top
computing firm. You had 15 minutes to work it out, or you didn't get
through to the 2nd phase:
4 men need to cross a bridge. There is 1 torch which must be with the
crossing men/man at all times, and only 2 men maximum can be on
the bridge at any time. Due to massive differences in fitness, the men
will take different lengths of time to get across the bridge:
Mathew = 1 minute
Mark = 2 minutes
Luke = 5 minutes
John = 10 minutes
Only the slower mans time counts eg. if Mark & John cross together
they will take 10 minutes to get to the other side, not 12.
You have 17 minutes to get all the men across - you can't throw the
torch back to the men on the far side; don't try leaving men part way;
leaving John to die in the wilderness isn't allowed either. - this isn't a
trick question, it's just possible.
I hope I explained it well enough.
Enjoy,
Jon (nb. it took me several hours and my mother 5 minutes)
I've heard this problem set up different ways before. For example: A man is carrying a fox, a
goose, and a bag of corn to the fair. He comes to a bridge which is so narrow that he can only carry
one item across at a time. If he leaves the fox and the goose together on one side, the fox will
eat the goose. If he leaves the goose and the corn together, the goose will eat the corn. How does
he get all three across intact?
Another variation of the same problem I have heard: Three men who can't swim come upon a river.
They find a boat on their side of the river with a maximum capacity of 250 lbs. The three men
weigh 100, 150, and 250 lbs. The boat must be occupied by at least one man to make it across
the river. How do they all get across?
1. The fox and the corn don't interact if left alone. So he takes the goose across, nothing
back, the corn across, the goose back, the fox across, nothing back, the goose across.
Alternatively swap 'fox' and 'corn' and the result is the same.
2. The 100 and 150 lb men go across, the 100 lb man goes back, the 250 lb man goes across,
the 150 lb man goes back, the 100 and 150 lb men go across again. Alternatively swap '100
lb man' with '150 lb man' and the result is the same.
I know another one similar to problem 1., and it goes like this: 3 vegetarians and 3 cannibals
need to cross a river in a boat that can only carry 2 people at once. The cannibals are very
hungry, but will only attack if they outnumber the vegetarians on either side of the river while
the boat is crossing the river. You are a local chieftain. You are old and infirm, and have no
desire to cross the river yourself, but you have the respect of the 6 people who do want to
cross the river and you want them all to cross in safety. What do you tell them to do?
Mathew and Mark really love to cross that bridge. They each do it twice.
Step1: Mathew & Mark = 2 mins
Step2: Mathew goes bac k= 1min (could be switched with step 4)
Step3: Luke & John = 10 mins
Step4: Mark goes back = 2 mins (or see step 2)
Step5: Mathew & Mark = 2 min
Total = 17 mins.