Four people, one flashlight, one narrow bridge, and 17 minutes to get everyone across
Four people need to cross a rickety bridge at night, which can hold at most two people at a time, and they need their one flashlight for every crossing. They cross at the slower person's pace: 1, 2, 5 and 10 minutes respectively. Can all four get across in 17 minutes or less?
Reveal the answer
Yes. Send the 1 and 2 across together (2 minutes), send the 1 back with the flashlight (1 minute, total 3). Send the 5 and 10 across together (10 minutes, total 13). Send the 2 back with the flashlight (2 minutes, total 15). Finally send the 1 and 2 across together again (2 minutes, total 17). The trick is sending the two slowest people together rather than one at a time, so their combined slowness is only paid once.