Question : Four people need to cross a narrow bridge at night.The bridge will only hold a maximum of two people. A torch must be carried by someone crossing the bridge – else they will fall in! But there is only one torch.
One person takes 10 minutes to cross, another 5 minutes, another 2 minutes and another 1 minute.
When two people cross the bridge together, they must go at the speed of the slowest.
You have to get all four people across in 17 minutes.
(e.g. First the person who takes 10 minutes crosses with the person who takes one minute, time so far = 10 minutes. Then the one-minute person goes back, total time = 11 minutes. Then the one-minute person crosses with the 5-minute person, total time = 16 minutes. One minute person crosses back, total time = 17 minutes. Last two people cross, total time = 19 minutes. Ooops!).
Answer :
i) First two people to cross are the one-minute & two-minute people. Time=2 mins.
ii) Two minute person goes back. Total time=4 mins.
iii) Five & Ten minute people go across. Total time=14 mins.
iv) One minute person goes back. Total time=15 mins.
v) One & two minute people go across – finished. Total time=17 mins.