Four friends on a hiking trip come across a bright they need to cross. The bridge is old and narrow, and thus it only permits two people to cross at a time. It is dark out and a flashlight is needed to cross the bridge, but the group only has one flashlight.
Each person crosses the bridge at different speeds and when two go together the faster one needs to adapt for the slower one (otherwise they can't share the light). The first person, Greg, needs 10 minutes to cross the bridge. The second, Marcia, needs 5 minutes. The third, Peter, need 2 minutes, and the fastest one, Jan, needs only 1 minute.
What is the shortest time that the group can take to cross the bridge?