Prisoners in tunnel
Four prisoners are planning an escape. They have a long and dark tunnel that can be used only with a flashlight and no more than two people can be in the tunnel at any time. They have a single flashlight.
Due to their varying physical abilities each prisoner needs a different time to pass through the tunnel. For prisoner A, B, C and D the time is 5, 10,20 and 25 minutes, respectively. (Of course they can go slower, if needed). A guard is coming every 61 minutes to check on them. Can all of them leave the tunnel before the guard discovers the escape?
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1 solution
The simplest way is for A+B to go first, A returns with the flashlight, A+C goes next, A returns, and finally A+D goes. This requires 10+5+20+5+25 =65 minutes. It does not work.
A better way is for A+B to go first, A returns and C+D goes. At this point B is already at the other side and B returns with the flashlight. Finally A+B goes again. This requires 10+5+25+10+10=60 minutes. They can do it this way! Notice that C's time does not matter.