I can only try –
re: the “brain wave cd” puzzle, if your referring to Feivel’s coin puzzle on page 1, this is different (and not the answer your gave in your last response.) And the response requires no luck.
re: the “15 gold pieces”, your answer is correct; I have another solution though:
1. Put 7 gold bars on each side. If they are equal in weight, the one left out is the light one. If unequal, go to step two.
2. Take the gold bars from the lighter side and put three on each side. If they are equal in weight, the one left out is the light one. If unequal, go to step three.
3. Take the gold bars from the lighter side and put one on each side. One will be the light one, or if they are equal in weight, the one left out is the light one.