Sunday, August 26, 2012

4th Grade Math?

I wave my white flag.

My son has these "problems of the week"  due every Friday.

This is his first one for the year.  Can you solve it?

The Bridge Problem

Four people need to cross a bridge at night.

The bridge is only strong enough to hold at most two people at once.

Because it is night, a flashlight must be used for all crossings.

It takes person A ten minutes to cross, person B five minutes, person C two minutes, and person D one minute.

If two cross at the same time they must walk at the slower man's pace.

How can you get everyone across in 17 minutes?

Oh, and there's only one flashlight.


  1. I cannot figure out how to do it in less than 19. So I've got that going for me.

  2. Ok. New answer. A & B walk across the bridge. 10 min. A is slow, but he's got a good arm, so he throws the flashlight over the ravine/river/whatever and C catches it. C & D then walk across the bridge in 2 minutes.

    12 minutes total.

  3. C and D cross. [2 min]. D returns [1 min].

    A and B cross [10 min]. C returns [2 min]

    C and D cross [2 min]

    That should equal 17 right? 2 + 1 + 10 + 2 + 2?

  4. Google.


    C&D cross. D goes back. A&B cross together, minimizing transit time across the bridge. C goes back. C&D cross together again

  5. Rats, I was hoping I could post the answer first.

    Anyway, I found this site on Google for the answer, and hopefully it'll come in handy for future problems.