John the jeweler is planning to make bangles from solid 14 karat gold. As shown below, his design will:

  • be available in 5 different diameters (D)
  • have a height (h) of 1 inch for all bangles
  • have an inside profile that is straight
  • have an outside profile with a radius of curvature (r) equal to D/2

John has fabricated a size XS prototype and found that it weighs 120 grams. John knows the density of 14 karat gold is 14 grams/cc, but is not sure how to calculate the weights of the other four bangles. He needs this information to determine the fabrication costs.

Can you help John by providing the weights of the other bangles with larger diameters?

This may look like a math textbook problem, however it is a variation of a classic geometry puzzle.


  • 2
    $\begingroup$ I'm currently downloading SolidWorks, brb with answers $\endgroup$ Commented Apr 10, 2015 at 8:06
  • $\begingroup$ What is $A$ that you've marked on the left-most diagram? $\endgroup$ Commented Apr 10, 2015 at 11:58
  • 1
    $\begingroup$ @Ian MacDonald - That is a convention to indicate the direction of View A-A. I should have made it look different from the other variables. $\endgroup$
    – Len
    Commented Apr 10, 2015 at 16:44

1 Answer 1


They are exacty the same so also 120 gram.

By making the diameter bigger you also make the ring narrower. And these cancel eachother out.

A better explanation here


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