Crossing the Bridge Puzzle

Puzzle: Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge?

wooden_bridge

Puzzle Solution:

It is 17 mins.
1 and 2 go first, then 1 comes back. Then 7 and 10 go and 2 comes back. Then 1 and 2 go again, it makes a total of 17 minutes.

36 Thoughts on “Crossing the Bridge Puzzle

  1. isn’t it 10 mins as the person who takes 10 min to cross, start with torch in hand and with him persons those take 1,2 and 7 mins go one by one.

  2. Priyanga.g on June 20, 2014 at 12:18 pm said:

    10 Mins is enough those 4 persons are cross the bridge.
    (10Mins=7Mins+2Mins+1Mins)

  3. 17 is not correct as it does not take into consideration time for return back. If 1 & 2 go and 1 come back then it is 3 mins, then 1 & 7 go (additional 7 mins) and 1 come back (add 1 min) then 1 & 10 go so additional 10 mins. Total 21 mins thats lowest

  4. Shahan on July 11, 2014 at 1:13 pm said:

    1 & 2 Crosses Time –> 2Min
    1 return Time –> 1Min
    3 & 4 Crosses Time –> 10Min
    2 return Time –> 2Min
    1 & 2 Cross Time –> 2Min
    Total Time –> 17Min

  5. Achint Mehta on August 13, 2014 at 12:59 pm said:

    ROFL

  6. Times for each person: A(1 min), B(2 mins), C(7 mins) and D(10 mins.)

    A&B > 2 mins
    A goes back 7 Mins
    A goes back 10 mis

    Total: 21mins

  7. Deepak Dadasaheb Yewle on December 5, 2014 at 3:39 am said:

    And she is from SPIT

  8. Lydia Komban Kunjumohan on December 26, 2014 at 11:00 am said:

    10min is only needed ,assuming the range as entire bridge first the one min guy as well as the 10 min guy cross as the 1 min guy crosses he holds torch so others can see d light now there is only 1 person so the 7 minb guy can cross,he completes then can the other 1 min with the 10 min guy so it can take only 10 min

  9. SHAKEEB NADEEM on January 27, 2015 at 1:35 am said:

    not mentioned that all people standing on same side
    assume 2 people standing on one side and 2 on other side

  10. Santosh on March 11, 2015 at 2:45 pm said:

    If it is tough to cross without one.. It would be still be tough for the one who returns..

  11. sharmi on March 18, 2015 at 5:09 pm said:

    It is 21mins.
    UP: (1,2) goes first. total=2mins.
    RETURN: (1) comes back. so total= 2+1=3mins.
    UP:(1,7) goes next. so total=3+7=10mins.
    RETURN:(1) comes back. so total= 10+1=11mins.
    UP:(1,10) goes next. so total= 11+10 =21mins.

  12. This is the actual question..

    Four people need to cross a rickety rope bridge to get back to their camp at night. Unfortunately, they only have one flashlight and it only has enough light left for seventeen minutes. The bridge is too dangerous to cross without a flashlight, and it’s only strong enough to support two people at any given time.

    Each of the campers walks at a different speed. One can cross the bridge in 1 minute, another in 2 minutes, the third in 5 minutes, and the slowest camper takes 10 minutes to cross. How can the campers make it across in exactly 17 minutes?

  13. so wts d final ans?? 17 or 21 mns???

  14. MAY I THINK 12 MNTS BCZ 1AND 2 ND PEOPLE GO AND THEN 37AND 10 TH ONE GO WILL RETURN ONE PERSON IN THAT BRIDGE THENH ALL PEOPLE WILL SAFELY REACH THERE

  15. albert john on September 15, 2015 at 2:50 pm said:

    i don’t know if these people above are fools or not. it is explained well above and again they are taking 21 min
    the ans is
    1 & 2 : 2min
    1come back : 1min
    10 & 7 : 10min
    2 come back: 2min
    1 & 2 : 2min
    TOTAL 17min

  16. 17 is the right answer :-

    First 1 and 2 will go – 2 minute.
    then 1 come back with torch- 1 minute
    then 7 and 10 will go with torch- 10 minutes
    then 2 will come back with torch- 2 minutes
    then 1 and 2 will go back – 2 minutes

    Total :- 2+1+10+2+2 = 17

  17. Nikesh sharma on November 16, 2015 at 4:11 pm said:

    All four should go at once, Let bridge decide will fall or not *Jako rakhe saiyaan mar sake na koi#

  18. purushotham tammali on December 11, 2015 at 11:38 pm said:

    I feel that answer does not suit the frameset of the question. It says a single person cannot cross bridge but while returning a single person is coming back…………..

  19. I like it people challenge it with 10 min. But you have to remember torch has light towards one direction. Hence question to challenge 10min logic starts with, Who holds the torch when 1min guy and 10 min guys starts crossing? in both cases, we some one crossing bridge without torch light at some point.

    Agree with me?

  20. Required time is 10 mnts only,

    First 10min guy & 7min guy starts- Tourch with 10 min guy,
    As soon as 7min guy reaches the other end 2 min guy will start (10 min guy is still in bridge with tourch and with 3 min left to finish his task), as soon as the 2min guy crosses (10 min guy still on bridge with 1min left and tourch in hand) 1min guy will start and finishes with the 10min guy. All in one end within 10min. :p

    • okay
      But when 10 min guy reaches at the point which is 3 min away from other side he must be facing his torch in the direction in which he is traveling. so it is not possible to other guys to on come on bridge because they won’t be able to see!!

  21. Rohit Kumar on April 5, 2016 at 12:02 pm said:

    1 & 2 Crosses Time –> 2Min
    1 return Time –> 1Min
    3 & 4 Crosses Time –> 10Min
    2 return Time –> 2Min
    1 & 2 Cross Time –> 2Min
    Total Time –> 17Min

  22. fdgfd on May 4, 2016 at 1:41 pm said:

    nice!!!

  23. Samir Meher on November 21, 2016 at 12:22 pm said:

    I was wondering one thing: if it is dangerous to cross the bridge without one, then how come every time one of the guys is returning from the other end. Mathematics is ok, but unable to conclude this statement..

  24. Makarand Dhuri on March 6, 2017 at 6:06 pm said:

    Ans can be 16 also.
    1)1&2 Go>> Once 1 reaches other side, 2 Returns back, Total step time=1+1=2
    2)2&7 Go>>Once 2 reaches other side, 7 Returns back, Total step time=2+2=4, Total Time=6.
    3)Lastly, 7 and 10 Goes, Reaches other side in 10 min, Total Time =2+4+10.
    So 16 can be answer, as it satisfies all the conditions.

  25. Sovi Sta on June 11, 2017 at 10:44 pm said:

    First 1 and 2 will go: 2 minute.
    then 2 come back with the torch: 2 minute
    then 7 and 10 will go with the torch : 10 minutes
    then 1 will come back with the torch: 1 minutes
    then 1 and 2 will go back: 2 minute
    Total : 17 mins

    First 1 and 2 will go: 2 minute.
    then 1 come back with the torch: 1 minute
    then 7 and 10 will go with the torch : 10 minutes
    then 2 will come back with the torch: 2 minutes
    then 1 and 2 will go back: 2 minute
    Total : 17 mins

    First 1 and 2 will go: 2 minute.
    then 1 come back with the torch: 1 minute
    then 1 and 10 will go with the torch : 10 minutes
    then 1 will come back with the torch: 1 minutes
    then 1 and 7 will go back: 7 minute
    Total : 21 mins

    First 1 and 10 will go with the torch:
    After 1min: 1 reaches and 10 has 9 minutes to go 1 minute
    Next 2min: 2 reaches and 10 has 7 minutes to go 2 minutes
    Next 7min: 7 reaches and 10 reaches the destination 7 minutes
    Total : 10 mins

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