River Riddle

awesome river
River Riddle - 30 May

Four people need to cross a dark river at night.They have only one torch and the river is too risky to cross without the tourch. if all people cross simultanoesly then torch light wont be sufficient.Speed of each person of crossing the river is different.cross time for each person is 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the river ?

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14 comments:

  1. 17 minutes

    1 and 2 up....1 minute back = 3 minutes
    7 and 10 minute up- 2 minute back= 12 minutes
    1 and 2 minute up = 2 minutes

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  2. If all cross, the light is not sufficient, but the riddle doesn't say anything about 3 of them not being able to cross. Assuming 3 ppl can cross at the same time... 10, 7 and 1 cross together in 10 min. 1 goes back in 1 min and picks up 2, crossing in 2 min. This is a total of 13 min.
    - DG

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    Replies
    1. VSR RAO 09701369409June 23, 2012 at 11:15 PM

      nice, me too but torch is not sufficient for 4 people but one who is capable can lead and may cover in 10m maximum at anybody'S point of crossing time.

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    2. I agree, the riddle should specify the number of people can cross the river simultaneously.

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    3. exactly what I thought when I did it, as the puzzle didn't specify the max. no. of people except that it has to be fewer than 4, thus 10+1+2=13 should be the shortest time

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  3. The initial solution most people will think of is to use the fastest person as an usher to guide everyone across. How long would that take? 10+ 1 + 7 + 1 + 2 = 21 mins. Is that it? No. That wouldmake this question too simple even as a warm up question.
    Let's brainstorm a little further. To reduce the amount of time, we should find a way for 10 and 7 to go together. If they cross together, then we need one of them to come back to get the others. That would not be ideal. How do we get around that? Maybe we can have 1 waiting on the other side to bring the torch back. Ahaa, we are getting closer. The fastest way to get 1 across and be back is to use 2 to usher 1 across. So let's put all this together.
    1 and 2 go cross
    2 comes back
    7 and 10 go across
    1 comes back
    1 and 2 go across (done)
    Total time = 2 + 2 + 10 + 1 + 2 = 17 mins

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    Replies
    1. Or... Person 1, 3 and 4 cross together, taking 10 minutes, person 1 then crosses back in 1 minute. Person 1 and 2 cross back over in 2 minutes.

      10+1+2=13

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  4. 10,7 and 1 go together in 10m and 1 will be back & 1 and 2 go in 2m.

    10+1+2= 13m

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  5. 10min f they all go together

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  6. DOESNT FUCKING SAY 2 PEOPLE ARE ALLOWED TO CROSS AT THE SAME TIME.
    FUCKING STUPID DESCRIPTION!

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