The people are of differing ages and fitness. The times it takes each person to cross individually are 20, 21, 23, 27, 29, 31, 34, 37, 41, and 43 minutes respectively. Each person knows everyone's time.
They have just 2 small flashlights, and each one casts only enough light to allow two people to cross safely. Whenever 2 people cross together, they cross in the time of the slower person.
The flashlights can be handed off at either end, but not part way. Crossers never turn around or stop on the bridge.
Everyone can get across in 3 hours (180 minutes) or less. How is this accomplished?