You have nine pearls, one of which is (as is usually the case in these problems) fake. You know that the fake pearl weighs less than the others, but it is (of course) impossible to distinguish from the others in any other way.
What is the minimum number of weighings that must be performed to find the fake pearl? How would you go about it?
OK, without looking at what anyone else has posted, I reckon it must be
two weighings. Weigh two groups of three, this will tell you which
group the light one is in (bearing in mind that if the two groups
balance, then you automatically know the dodgy pearl is in the third
group); then weigh two of the suspect group against each other
(similar story - should they balance, then the other one can be
declared fake without having to weigh it).
Solution
