4 people need to cross a narrow, dangerous bridge late at night. They have a total of 1 flashlight between them. Only a maximum of 2 people can cross the bridge at any one time. With such a dangerous bridge, the people crossing the bridge must always have the flashlight with them. Each person moves at a different speed, and thus any two people must travel at the speed of the slowest. The respective speeds of the 4 people are 10mins, 5mins, 2mins and 1min. What is the shortest time that everyone can safely cross the bridge. Before you look at the answer, it is less than 19 mins, but why?