Teaser Of The Day No. 238

[Carl]

Four people arrive at a river with a narrow bridge that can only hold two people at a time. It's nighttime and they have one torch that has to be used when crossing the bridge. Person A can cross the bridge in one minute, B in two minutes, C in five minutes, and D in eight minutes. When two people cross the bridge together, they must move at the slower person's pace.

Can they all get across the bridge in 15 minutes or less ?


Answer will be provided next Thursday evening after 21:00.

[elcarretero]

If the throw the torch yes, if not, no

[Carl]

ANSWER

Yes, they can cross in exactly 15 minutes. The group of four must follow these three steps.

1. First, A and B cross the bridge and A brings the light back. This takes 3 minutes.
2. Next, C and D cross and B brings the light back. This takes another 10 minutes.
3. Finally, A and B cross again. This takes another 2 minutes.