Puzzles & Games Time across the bridge-Solved

Discussion in 'Puzzles & Games' started by helenheaven, Sep 11, 2004.

  1. helenheaven

    helenheaven Premium Member

    4 people need to cross a narrow, dangerous bridge late at night. They have a total of 1 flashlight between them. Only a maximum of 2 people can cross the bridge at any one time. With such a dangerous bridge, the people crossing the bridge must always have the flashlight with them. Each person moves at a different speed, and thus any two people must travel at the speed of the slowest. The respective speeds of the 4 people are 10mins, 5mins, 2mins and 1min. What is the shortest time that everyone can safely cross the bridge. Before you look at the answer, it is less than 19 mins, but why?
  2. Kellter

    Kellter Member

    I figured it out to exactly 19 mins. Tryin to figure out how it could be less.
  3. ilovepizza

    ilovepizza Premium Member

    It takes exactly 12 minutes. Everyone takes off their shirts and unravil it. They tie all the unraviled stuff together to make a long string. They put the string through the little hole on the flashlight that you use to hang it with. They make a knot big enough so that it does not come out. The person who takes 10 minutes walks with the person who takes 5 minutes. Then one of the 2 people left behind pull the string untill the flashlight comes back to them. Now they both walk across taking 2 minutes.
  4. helenheaven

    helenheaven Premium Member

    It takes 17 minutes, but how ?
  5. pineappleupsidedown

    pineappleupsidedown Premium Member

    I liked ILovePizza's answer, haha. *trys to figure out 17*

  6. ilovepizza

    ilovepizza Premium Member

    The only way I can see it taking 17 minutes is if you do not count the time it takes the person who walks it in one minute to go back twice with the flashlight. Since that person has already crossed the bridge he is returning therefore not adding any time to crossing the bridge.
  7. Cinderloft

    Cinderloft Premium Member

    ilovepizza: i agree with you. counting that time, I get 19 minutes. any less and the people are either not counting the time it takes for minute man to cross, or they are tossing the flashlight back and forth. But with the given math and procedure, 19 is the shortest time possible. If they tossed the flashlight, then it would be 10+5+2+1 = 18. Cant wait to see the answer.

    Edit: They could have gone 2x2 and tossed the flashlight. 10 and 5 for 10, and 2 and 1 for 2. That would leave 12. Thats about it.
  8. helenheaven

    helenheaven Premium Member

    ready for the answer ? let me know and I'll post it
  9. Cinderloft

    Cinderloft Premium Member

    I know why now, it just hit me.

    They are not all crossing the same way.

    10 and 5 going one way with the flashlight, they cross in 10 minutes. They give the flashlight to 2 and 1, they cross in 2 minutes for a total of 12 minutes. Am I right?
  10. helenheaven

    helenheaven Premium Member

    1min & 2min cross the bridge first making a total of 2 minutes.

    1 min comes back = 3 minutes

    5 min and 10 mins cross = 13 minutes

    2 minutes goes back to get 1 min = 15 minutes

    2 minutes and 1 min cross = 17 minutes
  11. Cinderloft

    Cinderloft Premium Member

    Yes, that works too. :) Good job helen, I must say I was stumped.