There are 4 people who need to cross a footbridge at night. The bridge can only bear the weight of 2 of them at once. They only have one torch between them and it is too dark to cross without it. If two of them cross together they have to travel at the rate of the slowest person. One of them takes 1 minute to cross, another of them takes 2 minutes, the third takes 7 minutes and the last 10 minutes. The easiest way of tackling this is to list out all the possibilities systematically.
What's the quickest way for them all to cross?
Sorry, I'm not going to tell you the asnwer to this one! Suffice it to say that if you've come up with 21 minutes, that's wrong. It's possible for them all to cross in less than 20 minutes.
The easiest way of tackling this is to list out all the possibilities systematically.