Many years ago someone gave me the following puzzle. I thought that it seemed fairly easy, and worked out the answer, only to be told I was wrong. Took me the best part of a weekend to work it out in the end. How quickly can you do it?
There are 4 people who need to cross a river, using a row boat. The boat can hold at most 2 people, but can also be rowed by one person. There is one seat in the boat, so if two people are in the boat they must sit side by side, each using one oar. Each of the four people row at different speeds, so they must row at the speed of the slowest person. The people can row across in 1, 2, 5 and 10 minutes respectively. What is the shortest time that all 4 people can get across the river in?
For the bloody minded people who instantly say “Just get them to swim across”, assume that the river is chock full of ravenous piranha, and that swimming is a one way trip to being eaten alive. For the bloody minded people who try and fit more than two people in the boat: do that and it will sink, giving the piranha a feast. For anyone suggesting that they should look for a bridge, or build a bridge, or anything like that: there are no bridges, nothing to build bridges from, just row across already dammit…