You have six balls - three red and three black. All the red balls weigh differently, i.e. one of them is heavy, the other medium, and the third light. Each red ball has a black twin of the same weight. A heavy and a light ball put together weigh as much as two medium balls.
What is the least number of weighings required on a balancing scale to determine which is which?
You can sort the red balls in three weighings.
You can then compare the middle red to two of the black balls, in two more weighings, after which all is determined.
So it can certainly be done in 5 weighings, although I sincerely doubt that this is the best that you can do.
Less trivial boundary
