Play · Tricky
Torch on the bridge
Four people must cross a narrow bridge at night. It holds two at a time, and their one torch has to go on every crossing. Alone they take 1, 2, 5 and 10 minutes. A pair walks at the slower pace.
The puzzle
Start
Far side
0minutes
Tap one or two people on the torch's side, then cross.
How to think about it
Distrust the obvious move
The obvious plan is to let the fastest person walk everyone over, one at a time. That takes 19 minutes. The waste is that the 5 and the 10 each cost their full time on separate trips. Send them together, and make sure someone quick is already on the far side to bring the torch back.