← | 083 - Shelve the Books | 084 - Crossing the River | 085 - More Noodling Around | → |
Crossing the River (Four Cross the River in the UK version) is a puzzle in Professor Layton and the Unwound Future.
Puzzle
- US Version
Four police officers are attempting to chase a criminal across a river. However, their boat can hold only two people at a time.
The first trip across takes one minute, but since the rower gets tired, the second crossing takes two minutes if the same person rows, the third takes three, and so on, getting slower and slower each time the same person rows. In minutes, what's the shortest amount of time it will take the four police officers to get across the river?
- UK Version
Four police officers, A, B, C and D, are chasing a criminal across a river. However, their boat only holds two people, so they can't all cross at once.
It takes one minute for a police officer to row across, but after the first crossing he gets tired, so the second crossing takes two minutes, the third crossing takes three minutes, and so on.
What is the minimum number of minutes it would take for all four officers to cross the river?
Hints
Solution
Incorrect
- US Version
Too bad!
Try drawing a chart to help you figure it out.
- UK Version
Bad luck.
Drawing a diagram may prove helpful here.
Correct
- US Version
Correct!
It will take six minutes.
A rows A and B across. B rows back, picks up C, and C rows across. Now that A, B, and C are across one of them has to row back and get D, who then takes over and rows across.
- UK Version
That's right!
Six minutes. A and B go over on the first crossing, with A rowing. B rows the boat back. C then joins B in the boat and C rows across. A, B and C are now on the opposite side.
One of them needs to row back, but they have all rowed across the river once, so it doesn't make any difference who goes. Once one of them has rowed back, D gets in and rows to the other side.
It therefore takes six minutes to get everyone across.
Professor Layton and the Unwound Future | Puzzles in||
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