>Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge?<
17 minutes, if you can stipulate that at the beginning of the problem, the 10 minute and 7 minute people are on opposite sides of the bridge, each paired with one of the other two guys.
18 minutes, if they must all start on the same side of the bridge, but there is no requirement for all of them to end up on the opposite side.
This also assumes that person who can cross the bridge in as little as one minute has no problem slowing down to accommodate someone else's slower pace; otherwise there's no real point to trying to share the torch.
17 minutes, if you can stipulate that at the beginning of the problem, the 10 minute and 7 minute people are on opposite sides of the bridge, each paired with one of the other two guys.
18 minutes, if they must all start on the same side of the bridge, but there is no requirement for all of them to end up on the opposite side.
This also assumes that person who can cross the bridge in as little as one minute has no problem slowing down to accommodate someone else's slower pace; otherwise there's no real point to trying to share the torch.