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  • There are 4 people that must cross an old dilapidated bridge at night, All four walk at different speeds:

    • There is a young teenager that can cross the bridge in 1 minute.

    • There is teenager's older brother can cross the bridge in 2 minutes.

    • Their father can cross the bridge in 5 minutes.

    • Their grandfather can cross the bridge in 10 minutes.

  • Only two people can cross at a time otherwise the bridge will break.

  • Also they have an oil lamp that will only last 17 minutes

  • Any party that crosses the bridge (only one or two people) must have the oil lamp with them.

  • If two people cross together, they have to walk at the speed of the slower person.

How can the entire group cross the bridge in 17 minutes?

The best answer I got was:

  1. The teenager and the grandfather walk across the bridge with the oil lamp: 10 minutes

  2. The teenager walks back with the oil lamp: 1 minute

  3. The teenager and the father walk across the bridge with the oil lamp: 5 minutes

  4. The teenager walks back with the oil lamp: 1 minute

  5. The teenager and his older brother walk across the bridge with the oil lamp: 2 minutes

But that adds up to 19 minutes.

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  • 1
    $\begingroup$ Note that this is a pretty well-known puzzle, with its own Wikipedia page $\endgroup$ – GendoIkari Mar 23 '18 at 21:36
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They can cross in a total of:

17 minutes

Like this:

(To simplify, let's name the people A, B, C, and D, in order from fastest to slowest.)

  • A and B cross - 2 minutes, leaving CD|AB

  • A comes back - 1 minute, leaving ACD|B

  • C and D cross - 10 minutes, leaving A|BCD

  • B comes back - 2 minutes, leaving AB|CD

  • A and B cross - 2 minutes, leaving |ABCD

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  • $\begingroup$ Yes this makes sense! $\endgroup$ – user3011052 Mar 23 '18 at 21:53

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