Why do underground stations have three times more exits than entrances? 4 people want to cross a river. You have one barge that can hold no more than two passengers at a time. The first passenger rows at such speed that it takes him 1 min to cross the river. It takes the second 2 min, the third 5 min and the fourth 10 min. When there are two people in the barge, the slowest rower determines the speed at which they cross the river. What is the shortest amount of time in which you can get all 4 people to cross the river?
