Some Challenging logical questions

  1. Hm2

    Hm2 2

    Problems have been edited to be a bit more generic. The questions remains the same in spirit.

    1. You have been given an infinite number of ropes, each of which burns completely in exactly one hour. However, the rates at which different parts of any given rope varies. For example, if you cut a rope in half and burn one of those halves you may find it takes one minute. Or it might take 59 minutes. You can't predict how long it will take for a partial segment of a rope to burn to completion. How can you burn ropes, some perhaps simultaneously, so as to measure exactly one hour and 15 minutes?

    2. Devise a set of four test masses with integer masses that total to 40 kg such that the test masses can be used on a balance scale to measure the mass of any object with integral mass between 1 and 40 kg, inclusive.

    3. Using this image:
    Design a path through the lands so that you cross each of the seven bridges once and only once. The path must be uni-directional, i.e. you cannot double-back and move the way you came.
    Last edited by a moderator: Nov 8, 2011
  2. jcsd
  3. Hm2

    Hm2 2

    Can any1 be kind enough to help?
  4. gneill

    Staff: Mentor

    Your puzzles might fair better in the General Math Forum, as they don't appear to be specifically physics-oriented.

    Also, keep in mind that forum rules strongly discourage "bumping" a thread before 24 hours has elapsed.
  5. SammyS

    SammyS 9,249
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You have waited all of 5 minutes.

    Wait 24 hours before "bumping" your thread. That's one of the rules for these Forums.

    As to your 1st solution, you are not taking this statement into account, "Some parts burn faster than the others. "
  6. D H

    Staff: Mentor

    To Hm2: you posted this in physics homework; why?

    Some questions:
    1. Is this homework?
    2a. If the answer to question 1 is yes, why didn't you follow the template, and what made you think this is physics homework?
    2b. If the answer to question 1 is no, what made you post this here rather than in Brain Teasers?

    Eventually this thread needs to be moved to the Precalculus Mathematics homework section or to Brain Teasers.
  7. You will need to be able to place weights on either side of the balance to get all numbers.

    EDIT: But im trying to do this right now and I can't...haha

    EDIT2: Wait! If I break it in 4 different parts does that mean I break it 4 times, into 5 pieces?
  8. Do not post answers to these questions. They are the questions to an entry test.
    The guy is plagiarizing.
  9. D H

    Staff: Mentor

    Or not use some of the weights at all. For example, if one of the lopped-off chunks of stone has a mass of 1 kg, the remaining three chunks are not needed to weigh a 1 kg mass.
  10. D H

    Staff: Mentor

    Mentor's note
    The OP was indeed cheating and was plagiarizing as charged.

    These questions however are old and do not belong to the group that levied the charges. That group's specific wording and specific imagery does belong to them. I have rephrased the original post to get around the plagiarism problem. The OP has already received full answers to each of these questions elsewhere on the internet, so there is no need to worry that in answering the questions you would be aiding and abetting the OP's cheating.

    Feel free to have fun answering the questions as modified.
    Last edited: Nov 8, 2011
  11. D H

    Staff: Mentor


    1. This is relatively easy with three ropes. It can also be done with only two ropes. That solution isn't so easy.

    2. The goal is to come up with a set of four integer masses that total to 40 kg (example: 1 kg, 1 kg, 1 kg, and 37 kg, but that doesn't work) that will let you determine the mass of a given object whose mass is anywhere from 1 to 40 kg, in steps of 1 kg. Assume the object to be weighed is placed on the right hand pan of the balance scale. Each one of the test masses can be placed on the left hand pan, the right hand pan, or not used.

    3. The image that I supplied should be a dead giveaway.
  12. PeterO

    PeterO 2,319
    Homework Helper

    In that case..
    light both ends of a string - it will take 1/2 hour to burn
    At that time light both ends of a second string, plus one end of a third.
    When the second string has burned, light the other end of what is left of the 3rd sting, with "1/2 hour" of the string left, it will only take a further 15 mins to burn from each end.
  13. D H

    Staff: Mentor

    Now find a solution using only two ropes.

    Hint: Start by lighting one rope at one end. Wait until that rope burns to completion. Now do something with the other rope to mark the remaining time.
  14. D H

    Staff: Mentor

    The two rope solution:

    Light rope #1 at one end and let it burn to completion. This will happen at t=60 minutes. At this time, light rope #2 at both ends and somewhere in the middle. That internal burn will cut the rope into strands, each of which is burning at both ends. One strand will take 15-x1 minutes to completion, the other in 15+x1 minutes, where 0≤x1<15. That first strand will burn to completion at time t=75-x1 minutes. At this time, the other strand has 2*x1 minutes remaining before it burns to completion; light that other strand somewhere in the middle at this time. Now we once again have two strands each of which is burning at both ends. One strand will take x1-x2 minutes to burn to completion, the other x1+x2 minutes, where 0≤x2<x1. That first strand will burn to completion at time t=75-x2 minutes. At this time, the other strand has 2*x1 minutes remaining before it burns to completion; light that other strand somewhere in the middle at this time. Continue this process until both strands burn to completion at the same time (to within your ability to see any difference).

    Because 15>x1>x2>x3>...≥0, the sequence {75-x1, 75-x2, 75-x3, ...} will converge monotonically to 75.
  15. The first question is quite easy, it can be done with 2 ropes as well as 3.
    The second question definitely a trick question, as I myself have not found the four individual weight values.
    Same goes for the third. Can't figure it out, even with the above given diagram. :P
  16. D H

    Staff: Mentor

    Not at all a trick question!

    Each one of the individual weights can be on the left pan of the scale, the right pan, or not used. Three choices. What base does that suggest you should be working in?

    This one is a trick question. Click on the image. What is the name of the image file (the part after the last slash and before the .png suffix)?
  17. I fully agree with your method of shifting weights to the left and right side of the pan. But there must be a formula, through which we can determine the 4 integral weight values...or do we have to try out random weight value combinations again and again until we hit the right one?
  18. D H

    Staff: Mentor

    The selection of weights is anything but random.

    Another hint: If the object's mass could be negative, the correct selection of (positive) test weights would be able to handle everything from -40 kg to +40 kg, a total of 81 values.

    What is the prime factorization of 81? What base should you be working in? (Hint: It's not base 10.)
  19. This is the problem which im facing. Have spent 4 hours on this question trying out random weight combinations, but no luck. There are always 2 or 3 mass values which cannot be determined using the 4 weight combinations. This has led me to believe that this question cannot be solved using only 4 parts.
  20. -40kg? is that even possible?
  21. Explain the term 'base' to me please. :P
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