Brain Quiz #8

4 men approach a bridge at night. They have only one flashlight, and in order to cross safely, only 2 people can travel across at a time, holding the flashlight. They can also only travel as fast as the slowest person currently crossing. It takes Jon 1 minute to cross, Dan 2 minutes to cross, Mike 5 minutes to cross, and Al 10 minutes to cross. If they must have the flashlight in order to cross the bridge, what’s the shortest amount of time it would take to get all 4 guys across the bridge?

Difficulty Rating: 10

25 thoughts on “Brain Quiz #8

  1. 20 minutes, I think. But it seems too easy. Am I missing something?

    I’m assuming the flashlight has to be carried back. If they can tie a rope to it or something and drag it back across the bridge, then I’d say 12.

    Oh wait, possible trick: if Mike and Al are approaching from one side and the other two from the other, that would also make it 12.

    So do I get first correct answer and first two incorrect answers?

  2. You’re prolly gonna want to explain how you’re getting your answers as well, cause so far, I have no clue where you are getting these numbers from.

  3. To clear up any confusion:

    1.) The only way to get the flashlight from one side to the other is by walking with it. Thus they can not throw it, tie a rope and pull it, roll it, or any other thing that will make the flashlight go from one side of the bridge to the other without actually walking the thing.

    2.) There’s nothing special about the bridge, the place, the people, or something like that. This isn’t some trick question where the answer is to build a helicopter out of a paperclip and some dental floss or anything.

  4. in the chance that my answer wasn’t sufficient.

    jon walks al across and comes back. 12 minutes expended so far.

    jon walks mike across and comes back. 7 more minutes for a total of 19 minutes.

    jon walks dan across and everybody’s happy. 3 more minutes for a total of 22 minutes.

    sufficient?

  5. I would say the fastest time for all of them to get across would be 19 minutes.

    Jon would be the one carrying the flashlight back and forth across the bridge. So 10 minutes across the first time with Al, 1 minute to come back, 5 minutes across with Mike, 1 minute coming back, and finally 2 minutes across with Dan.

  6. 19 minutes. Jon and Al go across first, thats 10 minutes, then Jon takes the flashlight back across by himself, 1 minute, Mike and Jon then go across, that is 5 minutes, Jon, yet again goes across, 1 minute, finally, Jon and Dan go across, 2 minutes. So, 10+1+5+1+2=19 minutes to get everyone across….unless you have to add their times together, in which case it would be 22 minutes.

  7. 19 Minutes

    Jon and Al cross 10 minutes
    Jon goes back 1 minute
    Jon and Mike cross 5 minutes
    Jon goes back 1 minute
    Jon and Dan cross 2 minutes
    Now they are all across 10+1+5+1+2 = 19

  8. I could get it across with less than a paperclip and a dental floss.

    AFter all – I stopped an acid leak with chocolate bars, stopped a bomb with hockey tickets…. Yeah. I rock.

  9. 12 mniutes

    Al and Mike are on one side of the bridge, while Jon and Dan stand at the other side.
    Al and Mike cross in 10 minutes bringing the flashlight to Jon and Dan, who then use it to cross in 2 minutes.

    10 + 2 = 12 minutes

    The question does not say they all are going the same way across the bridge.

  10. More information:

    1. The 4 guys are all traveling the same direction. They all start on the same side of the bridge, and all must end up on the other side of the bridge.

    2.) If it makes it easier for you.. let’s pretend it’s Al’s Flashlight, and he refuses to go home without it…

    3.) McGuyver could probably create a flashlight out of Al, and cross in negative time. For the purposes of this question, McGuyver is not available.

  11. 1) #1 and # 2 go across (2 min)
    2) #1 comes back (1 min: total 3)
    3) #5 and #10 cross (10 min: total 13)
    4) # 2 comes back (2 min: total 15)
    5) #1 and # 2 go across (2 min: total 17)

    Or are you going to tell the slowest time.

  12. Well if it’s Al’s flashlight then he has to cross every time, whether to bring someone over, or bring back the flashlight, so the other guy’s speeds are not relevant.
    Al and Jon = 10 min
    Al goes back = 10 min
    Al and Dan = 10 min
    Al goes back = 10 min
    Al and Mike = 10 minute

    So we’re looking at 50 minutes, cause Al is scared of the dark.

  13. Okay hmmm… how about 5 minutes then

    Jon carries everyone over one at a time while the person he’s carrying holds the flashlight.

    So 1 minute there with Al on his back
    1 minute back
    1 minute there with Dan on his back
    1 minute back
    1 minute there with Mike on his back

  14. finally got it. ( your hints actually confused me ).

    jon and dan cross. dan comes back with flashlight. total time 4 minutes.

    mike and al cross. jon comes back. 11 more minutes for a total of 15.

    jon and dan cross for a final total of 17.

    whew.

  15. 5 minutes total:
    1 – Jon carrying Dan across
    1 – Jon returning
    1 – Jon carrying Mike across
    1 – Jon returning
    1 – Jon carrying Al across

    I know that doesn’t make any sense… boo to this one.

  16. Jon and Dan go across (2 min.) Dan comes back (2 min.) Al and Mike go across (10 min.) Jon comes back (1 min.) finally Jon and Dan go across again (2 min.) for a total of 17 min.

  17. 1. Jon and Dan cross together [2 minutes]
    2. Jon returns [1 minute. 3 minutes total]
    3. Mike and Al cross [10 minutes. 13 minutes total]
    4. Dan returns [2 minutes. 15 minutes total]
    5. Jon and Dan cross together [2 minutes. 17 minutes total]

    Total time: 17 minutes.

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