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There is a family which contains 4 members: Dad, Mom, Son, Daughter. They have a boat which carries only 100 kg at a time. Mom and Dad weigh 100 kg each, while Son and Daughter weigh 50 kg each.

How can they cross a river?

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    $\begingroup$ I downvoted this because it's too simple and easy, and therefore not a good puzzle IMO. $\endgroup$
    – user14478
    Nov 9, 2016 at 12:45
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    $\begingroup$ @LukasRotter For a moment I thought it was impossible (hence my slight delay in getting my answer posted, compared to the other two). $\endgroup$ Nov 9, 2016 at 12:54
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    $\begingroup$ They swim...... $\endgroup$ Nov 9, 2016 at 20:09

5 Answers 5

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Solution:

Both childs go in the boat to the other side. one comes with the boat. then goes one of the parent and the child who was staying on the other side comes with the boat and again go both childs and repeat for the other parent. last, both child go together and the 4 of them have crossed

C1(child 1) C2(child 2) M(mom) D(dad) B(boat)
M D ---------- C1 C2 B
M D C1 B------ C2
M C1 ---------- C2 D B
M C1 C2 B ----- D
M ------------ D C1 C2 B
M C1 B ------- D C2
C1 ------------ D M C2 B
C1 C2 B ------- D M
[ ] --------- C1 C2 M D B

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    $\begingroup$ Wow, you and hexomino answered at exactly the same second! $\endgroup$ Nov 9, 2016 at 12:39
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A possible way

Son and Daughter cross
Son comes back
Dad crosses
Daughter comes back
Son and Daughter cross
Son comes back
Mom crosses
Daughter comes back
Son and Daughter cross.

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    $\begingroup$ Wow, you and lois2b answered at exactly the same second! $\endgroup$ Nov 9, 2016 at 12:38
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    $\begingroup$ :O woooow thats inusual $\endgroup$
    – lois6b
    Nov 9, 2016 at 12:42
  • $\begingroup$ Wow, I don't think I've seen that happen before. $\endgroup$
    – hexomino
    Nov 9, 2016 at 12:46
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    $\begingroup$ @hexomino It's happened at least once before here. $\endgroup$ Nov 9, 2016 at 12:56
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    $\begingroup$ @lois6b based on your id answer-45482 i marked it yours as a answer. $\endgroup$
    – Kavin Smk
    Nov 9, 2016 at 12:59
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It can be done in a total of

five crossings one way and four crossings going back,

as follows.

  • Son and Daughter cross together, then Son goes back.

  • Mum crosses, then Daughter goes back.

  • Son and Daughter cross together, then Son goes back.

  • Dad crosses, then Daughter goes back.

  • Son and Daughter cross together.

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In general, the puzzle can be solved if every person on their own is light enough to cross, and if there is just a single person, or at least two persons who are light enough to cross together.

A possible solution, not necessarily optimal: The two light people A and B cross, A comes back, someone crosses on their own, B comes back, and we have the original situation with one person less on the left side. Hope A and B don't get tired.

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Son and daughter cross.
Son goes back with the boat.
Father crosses.
Daughter goes back with the boat.
Son and daughter cross.
Son goes back with the boat.
Mother crosses.
Daughter goes back with the boat.
Son and daughter cross.

Too easy.

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    $\begingroup$ It was also answered in the same way four weeks earlier... I don't think your answer adds anything that wasn't already in there does it? $\endgroup$
    – Chris
    Dec 5, 2016 at 23:38
  • $\begingroup$ I didn't check the other answers. I'm just solving and posting my answer. $\endgroup$ Dec 5, 2016 at 23:41
  • $\begingroup$ Well the good news is that you got it right! :) $\endgroup$
    – Chris
    Dec 5, 2016 at 23:43
  • $\begingroup$ This is (at least) the third time in a short interval that you've posted an answer to a puzzle for which an answer was already given and accepted as correct, some time ago. Welcome to Puzzling SE - you really should read through all the answers to a question before providing your own, to make sure you're not duplicating an answer already given. In particular, if an answer has already been accepted, adding a new answer should only be done if you have something unique and of great relevance to add. And answering old questions makes both of these way more important. Please, read before answering! $\endgroup$
    – Rubio
    Dec 6, 2016 at 1:08
  • $\begingroup$ Well, I'm currently banned from asking questions here, and I'm bored so I was trying to garner upvotes by solving puzzles. That's why I joined; for puzzle solving. I'll take a look at other answers though. $\endgroup$ Dec 6, 2016 at 1:11

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