It’s the Friday Puzzle…..



Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

The other day my friend sent me this note:

‘I’ve changed the calibration dial on my bathroom scale, and so the readings are off by a consistent amount. When I am on the scale it reads 170 pounds, and when my wife is on it, it reads 130.  When we stand on it together the scale reads 292 pounds. How should I adjust the scale?’

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

53 comments on “It’s the Friday Puzzle…..

  1. Stop worrying about how much you weigh and enjoy life. Throw your scales out of the window, and with a bit of luck it’ll land on an armadillo.

  2. Tom (iow) says:

    Relatively easy I would say. Got it in a couple of minutes.

  3. Eddie says:

    Easy one, but good.

  4. DiscoveredJoys says:

    Easy. Answer dropped out immediate I wrote the situation down algebraically.

  5. Anne Elk says:

    Got an answer. About two minutes.

  6. Shaun Bear says:

    You need to adjust the scale by the little twiddly knob on the bottom.

    It only took a couple of seconds to work out by how much.

  7. ctj says:

    about a minute. it must have been something your friend “ate.”

    • Slow Learner says:

      Only took me a minute ctj, just like you.
      This means that I can spend the rest of the day waiting for you to elaborate on your intriguing comment on Richard’s puzzle from 4th October 2103.
      In case you have forgotten it, here it is again:-

      OCTOBER 4, 2013 AT 10:40 AM
      there are two valid solutions.

      without explaining what is special about eric’s children, yes, parents of that kind of children really do think and talk that way.”

    • ctj says:

      richard’s solution to the problem of the ages of three children was 1,1, and 9. he incorrectly assumed that the parent of twins does not distinguish between the twins as older or younger. in truth, all parents of twins know who is older, even though the difference is often mere minutes. the distinction can lead to absurd results, particularly in cultures that place heavy emphasis on the oldest child. i know chinese parents of twins who actually admonish the “older” child of her responsibility to supervise and take care of her sister, who is a mere 6 minutes younger.

      of course, the 6-year-olds in the puzzle need not be twins. it is possible to have children who are slightly less than one year apart.

    • Slow Learner says:

      Thank you for this ctj.
      I think, however, that there is a problem with your answer. If you (and the Chinese parents) are going to measure ages to the nearest minute you need to determine how old all three children are to the nearest minute. These three ages will then, when multiplied together, generate the number 36. Good luck with that.
      In addition the sum of their ages will need to be “exactly the number of biscuits we just consumed.”
      In effect your solution uses two numbering systems – one to the nearest year and one to the nearest minute.
      I realise that I have not expressed this very eloquently, but I hope that you “catch my drift”.

  8. Okay got it, but used trial and error 🙂

  9. ScienceJufMeike says:

    Get off the scale and adjust it with nobody standing on it (as it should then give you the ‘off’ weight). No need for algebra (although, yes, it only takes about 2 minutes in this case – but then you still need to adjust the scale…..).
    And then throw it out the window. Horrible things, scales.

  10. mgm75 says:

    Once you realise the simple trick, that’s easy to work out. Got it within a couple of minutes!

  11. Roy Gillett says:

    About 10 seconds to come up with AN answer and another minute or so to work it through with the actual numbers to check it.

    • Bugs Man says:

      Roy, after re-reading the question, the same for me. What threw me was the word “calibration” which, by definition, requires at least 2 known points on a straight line graph through the origin, so I first tried some fairly complicated algebra in search of a percentage error. When that failed the penny dropped. Had the question stated “zeroed” rather than “calibrated” ……..

  12. Phil H says:

    Of course, being a modern man, I first of all converted the pounds to metric measures, then carried out the algebra, got the answer, then converted it back to pounds. Hmm, the answer ends in .99999 pounds. I wonder if that’s right.

  13. Stevie says:

    Took about five minutes to get to the answer. Even had to resort to doing it in excel.
    Tried to draw a graph but couldn’t find my scale rule!

  14. SofARMaths says:

    Algebrasic. About a min…

  15. Anonymous says:

    A minute or two, and a scrap of paper to work the algebra to solve for the unknown weights and offset.

  16. Duncan says:

    Took about a minute – my answer doesn’t look quit right though.

  17. Duncan says:

    Oops, got it now. Make that 5 minutes.

  18. stev says:

    Well, given that you have just changed the calibration I presume you are happy with the new setup.

  19. Lazy T says:

    Simple maths, then I suggest you adjust the scales with a sledgehammer.

  20. mittfh says:

    Evidently the problem was set in the US, given over here we either use Kilogrammes (if we want to be metric like our continental neighbours) or a combination of Stones and Pounds (1st = 14 lb) if we want to be truly British.

    Of course, adjusting the scales to give accurate results is completely different to adjusting the scales to give results both you and your wife are happy with… 😀

  21. Stewart says:

    5 minutes, but then I’d just fallen off my scale.

  22. Greetje says:

    Buy a new one i’d say …

  23. Stevenz says:

    I came up with my answer in less than a minute. Of course, I don’t know if my answer in right.
    (What I really like about the Friday puzzle is that I know which day of the week it is.)

  24. One Eyed Jack says:

    You adjust that scale and you’ll have a very unhappy wife.

  25. pete says:

    When no ones is on it, won’t the scale read 8?

  26. fotoflex2013 says:

    Tell your friend to adjust the scales so they reag 99 when his wife is on it.
    She’ll be sooo happy! 🙂

  27. fotoflex2013 says:


  28. Alma says:

    Please let’s have less maths problems and more logic puzzles! I can never do the maths ones, being thick at that subject. 😦

  29. Turd says:


  30. Questioman says:

    When 2 dogs and 1 snail are on the same scale, it reads 73 pounds.
    When 1 dog and 4 snails are on the scale, it reads 58 pounds.
    When 3 dogs and 6 snails are on the scale, it read 128 pounds.
    What is the weight of a dog?
    *Note- They are on the same scale, so you will need to solve the above puzzle first.

    • Lazy T says:

      Does it weigh the same as a duck?

    • Questioman says:

      As you should have already found out the scale is 8 pounds too high.
      Now for the answer to my question.
      If the scale is too high by 8 pounds, the correct reading should be –
      a) 2dogs 1snail = 65 pounds
      b) 1dogs 4 snails = 50 pounds
      c) 3dogs 6 snails = 120 pounds

      the solution to the question is
      1. Take (A+C)-(B+C) you will get the weight of 1dog and -3snails which is 15 pounds.
      2. Take C divide by 2 and you will get weight of 1.5dogs and 3 snails which is 60 pounds.
      3. Add up the above two steps and you will get the weight of 2.5dog which is 75 pounds.
      4. 75/2.5 and one dog weight 30 pounds.

    • ChrisR says:

      OK but a bit long-winded answer as there are only two variables:
      A: 2D + 1S = 65
      B: 1D + 4S = 50
      So 4A; 8D + 4S = 260
      and 4A – B : 7D = 210 => D = 30

    • Questioman says:

      Great, ChrisR, this question just came out of my mind from nowhere, thanks for a better answer. It’s seem much easier now.

  31. Anonymous says:

    Another easy-peasy if you did Grade-8 Algebra and paid attention!

  32. h3rne777 says:

    Trick question, people. The answer to “How should I adjust the scale?” is “To whatever negative off-set is necessary for your wife to believe she is her ideal weight”.

    • ctj says:

      or alternatively, to whatever offset causes your wife’s lie about her weight, based on what she sees on the scale, is in fact her true weight if the scale had not been adjusted.

  33. dayuntoday says:

    about 2.5 minutes. /nice

  34. dayuntoday says:

    Would have taken me about half a minute had I immediately resorted to algebra rather than trial and error. 🙂 And there are some great comments! 🙂

  35. Rudy says:

    Congrats to Chelsea!

  36. I do agree with all the ideas you’ve presented in your post. They’re really convincing and will certainly work. Still, the posts are very short for beginners. Could you please extend them a bit from next time? Thanks for the post.

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