Exploding Bridge

Q: There is a bomb on a bridge. It will go off in 17 seconds. There are four people on one side of the bridge. It is night time and they only have one flashlight. They need to cross to the other side of the bridge for safety. However, only two people can cross at one time while carrying the flashlight. Every time two people cross, one has to go back with the flashlight and two more can cross and so on until all of them are on the other side before the bomb explodes. One person takes 1 second every time he crosses the bridge once. The second person takes 2 seconds, another takes 5 seconds, and the last person takes 10 seconds to cross. In what order did they have to cross the bridge to survive before the bridge explodes in 17 seconds?

A: The person who takes 1 second to cross the bridge crosses with the person who takes 2 seconds. So two seconds already passed. The 1 second person goes back so now its three seconds that passed. He gives the flashlight to the person who takes 10 seconds and the person who takes 5 seconds to cross. So now its a total of 13 seconds that passed. The person who takes 2 seconds goes back and comes back with the one second person making the total time 17 seconds.