Brain Teaser: Making Ends Meet

11 Comments

1. B on March 26, 2008 at 9:11

   He needs to define what a loop is first.

   
   
   
2. zenkat on April 4, 2008 at 7:05

   Fun lit­tle prob­lem, but I have one minor quib­ble. Since the ini­tial selec­tion of an open or closed end is ran­dom, the answer is a sta­tis­ti­cal dis­tri­b­u­tion, with 160 loops as the expect­ed mean.

   Bonus ques­tion: what is the stan­dard devi­a­tion of the distribution?

   
   
   
3. Paul on April 11, 2008 at 12:59

   You can not base math­e­mat­i­cal log­ic on a human con­di­tion — Assump­tion. You’re argu­ment is valid, pro­vid­ed your assump­tion of the peo­ple will do, what you expect.

   
   
   
4. Govardhan on April 30, 2008 at 1:35

   i dont agree with, what Wes is saying…This ques­tion can have mul­ti­ple answers…

   
   
   
5. Sue on May 1, 2008 at 8:38

   Yeah..yeah
   Try this..2 loops for each knot tied and 1 loop that the ties create.

   
   
   
6. C on May 11, 2008 at 6:27

   B is right. What is a loop for the author?
   If I take two ropes and tie them one end to one end, I get a longer rope wit two ends free, Then I tie those two free ends and I get a big loop. Ecah per­son gets one big loop, which gives a total of 120 loops. What is wrong with this log­ic ver­sus the for­mu­la­tion of the problem?

   
   
   
7. CB on May 15, 2008 at 4:38

   I agree com­plete­ly with C. I don’t see the log­ic in attempt­ing to com­pli­cate a rather sim­ple solution.

   
   
   
8. J on May 15, 2008 at 5:02

   I agree with both B and C’s replies; the ques­tion was both unclear and com­pli­cat­ed. How­ev­er, I did like fig­ur­ing it out and came up with 120 loops, also.

   
   
   
9. Ali on May 17, 2008 at 1:31

   The answer giv­en does not address the prob­lem as ini­tial­ly set out, which was to start by tak­ing a ran­dom selec­tion of any two ends at which point you have a 50:50 chance of select­ing ends from two dif­fer­ent ropes and you end up with an aver­age of 180 loops. The giv­en answer only works if hav­ing already select­ed one of 4 avail­able ends you are asked to tie that to one of the reamin­ing 3.

   
   
   
10. Ali on May 17, 2008 at 5:49

    Cor­rec­tion, your solu­tion is the right one since the answer is the same whichev­er of the 4 ends is select­ed first.

    
    
    
11. paula on May 21, 2008 at 11:24

    i don’t think you can say just like that how many peo­ple will fin­ish tying the ropes…asumpting that evry­one man­ages to tie the first ends of the ropes and than the oth­er two,we get 2 loops for evry per­son ..wich means 240 loops…i mean we can only say wich is the max­i­mum num­ber of loops that the 120 per­sons can cre­ate and not how many of them will man­age to do what they’re asked