Originally posted by Mr. Belvedere
[B]Place 3 marbles on each tray.

If the first weighing doesn’t balance, remove all the marbles from the lighter side, and place one marble on each tray from the heavier tray. The heavier side is the heavier marble, but if they balance, then the marble from the heavier tray from the first weighing that was not weighed in the second weighing is the
heavier one.

If the marbles balance on the first weighing, remove the marbles from the trays, and place 2 of the remaining unweighed marbles on the trays, one on each tray. If one is heavier, it is the heavier marble, but if they balance, the remaining unweighed marble is the heavier one. [/B]
In exactly 30 minutes!
Now a real pain in the ass.
There are 12 marbles, 11 have the same weight and 1 has a different weight, but you don’t know if it’s heavier or lighter !!
There is a balance. You can weigh 3 times.
How do you have to weigh (in 3 times), to know wich marble has the different weight??
Time starts!!